diff --git a/doc/pyproject_toml.rst b/doc/pyproject_toml.rst
index 9620ce75..f6755a87 100644
--- a/doc/pyproject_toml.rst
+++ b/doc/pyproject_toml.rst
@@ -96,7 +96,8 @@ requires-python
   A version specifier for the versions of Python this requires, e.g. ``~=3.3`` or
   ``>=3.3,<4``, which are equivalents.
 license
-  A table with either a ``file`` key (a relative path to a license file) or a
+  A valid SPDX `license expression <https://peps.python.org/pep-0639/#term-license-expression>`_
+  or a table with either a ``file`` key (a relative path to a license file) or a
   ``text`` key (the license text).
 license-files
   A list of glob patterns for license files to include.
diff --git a/flit_core/flit_core/_spdx_data.py b/flit_core/flit_core/_spdx_data.py
new file mode 100644
index 00000000..5af71dbb
--- /dev/null
+++ b/flit_core/flit_core/_spdx_data.py
@@ -0,0 +1,651 @@
+# This file is generated from SPDX license data; don't edit it manually.
+
+licenses = \
+{'0bsd': {'id': '0BSD'},
+ '3d-slicer-1.0': {'id': '3D-Slicer-1.0'},
+ 'aal': {'id': 'AAL'},
+ 'abstyles': {'id': 'Abstyles'},
+ 'adacore-doc': {'id': 'AdaCore-doc'},
+ 'adobe-2006': {'id': 'Adobe-2006'},
+ 'adobe-display-postscript': {'id': 'Adobe-Display-PostScript'},
+ 'adobe-glyph': {'id': 'Adobe-Glyph'},
+ 'adobe-utopia': {'id': 'Adobe-Utopia'},
+ 'adsl': {'id': 'ADSL'},
+ 'afl-1.1': {'id': 'AFL-1.1'},
+ 'afl-1.2': {'id': 'AFL-1.2'},
+ 'afl-2.0': {'id': 'AFL-2.0'},
+ 'afl-2.1': {'id': 'AFL-2.1'},
+ 'afl-3.0': {'id': 'AFL-3.0'},
+ 'afmparse': {'id': 'Afmparse'},
+ 'agpl-1.0-only': {'id': 'AGPL-1.0-only'},
+ 'agpl-1.0-or-later': {'id': 'AGPL-1.0-or-later'},
+ 'agpl-3.0-only': {'id': 'AGPL-3.0-only'},
+ 'agpl-3.0-or-later': {'id': 'AGPL-3.0-or-later'},
+ 'aladdin': {'id': 'Aladdin'},
+ 'amd-newlib': {'id': 'AMD-newlib'},
+ 'amdplpa': {'id': 'AMDPLPA'},
+ 'aml': {'id': 'AML'},
+ 'aml-glslang': {'id': 'AML-glslang'},
+ 'ampas': {'id': 'AMPAS'},
+ 'antlr-pd': {'id': 'ANTLR-PD'},
+ 'antlr-pd-fallback': {'id': 'ANTLR-PD-fallback'},
+ 'any-osi': {'id': 'any-OSI'},
+ 'any-osi-perl-modules': {'id': 'any-OSI-perl-modules'},
+ 'apache-1.0': {'id': 'Apache-1.0'},
+ 'apache-1.1': {'id': 'Apache-1.1'},
+ 'apache-2.0': {'id': 'Apache-2.0'},
+ 'apafml': {'id': 'APAFML'},
+ 'apl-1.0': {'id': 'APL-1.0'},
+ 'app-s2p': {'id': 'App-s2p'},
+ 'apsl-1.0': {'id': 'APSL-1.0'},
+ 'apsl-1.1': {'id': 'APSL-1.1'},
+ 'apsl-1.2': {'id': 'APSL-1.2'},
+ 'apsl-2.0': {'id': 'APSL-2.0'},
+ 'arphic-1999': {'id': 'Arphic-1999'},
+ 'artistic-1.0': {'id': 'Artistic-1.0'},
+ 'artistic-1.0-cl8': {'id': 'Artistic-1.0-cl8'},
+ 'artistic-1.0-perl': {'id': 'Artistic-1.0-Perl'},
+ 'artistic-2.0': {'id': 'Artistic-2.0'},
+ 'aswf-digital-assets-1.0': {'id': 'ASWF-Digital-Assets-1.0'},
+ 'aswf-digital-assets-1.1': {'id': 'ASWF-Digital-Assets-1.1'},
+ 'baekmuk': {'id': 'Baekmuk'},
+ 'bahyph': {'id': 'Bahyph'},
+ 'barr': {'id': 'Barr'},
+ 'bcrypt-solar-designer': {'id': 'bcrypt-Solar-Designer'},
+ 'beerware': {'id': 'Beerware'},
+ 'bitstream-charter': {'id': 'Bitstream-Charter'},
+ 'bitstream-vera': {'id': 'Bitstream-Vera'},
+ 'bittorrent-1.0': {'id': 'BitTorrent-1.0'},
+ 'bittorrent-1.1': {'id': 'BitTorrent-1.1'},
+ 'blessing': {'id': 'blessing'},
+ 'blueoak-1.0.0': {'id': 'BlueOak-1.0.0'},
+ 'boehm-gc': {'id': 'Boehm-GC'},
+ 'boehm-gc-without-fee': {'id': 'Boehm-GC-without-fee'},
+ 'borceux': {'id': 'Borceux'},
+ 'brian-gladman-2-clause': {'id': 'Brian-Gladman-2-Clause'},
+ 'brian-gladman-3-clause': {'id': 'Brian-Gladman-3-Clause'},
+ 'bsd-1-clause': {'id': 'BSD-1-Clause'},
+ 'bsd-2-clause': {'id': 'BSD-2-Clause'},
+ 'bsd-2-clause-darwin': {'id': 'BSD-2-Clause-Darwin'},
+ 'bsd-2-clause-first-lines': {'id': 'BSD-2-Clause-first-lines'},
+ 'bsd-2-clause-patent': {'id': 'BSD-2-Clause-Patent'},
+ 'bsd-2-clause-views': {'id': 'BSD-2-Clause-Views'},
+ 'bsd-3-clause': {'id': 'BSD-3-Clause'},
+ 'bsd-3-clause-acpica': {'id': 'BSD-3-Clause-acpica'},
+ 'bsd-3-clause-attribution': {'id': 'BSD-3-Clause-Attribution'},
+ 'bsd-3-clause-clear': {'id': 'BSD-3-Clause-Clear'},
+ 'bsd-3-clause-flex': {'id': 'BSD-3-Clause-flex'},
+ 'bsd-3-clause-hp': {'id': 'BSD-3-Clause-HP'},
+ 'bsd-3-clause-lbnl': {'id': 'BSD-3-Clause-LBNL'},
+ 'bsd-3-clause-modification': {'id': 'BSD-3-Clause-Modification'},
+ 'bsd-3-clause-no-military-license': {'id': 'BSD-3-Clause-No-Military-License'},
+ 'bsd-3-clause-no-nuclear-license': {'id': 'BSD-3-Clause-No-Nuclear-License'},
+ 'bsd-3-clause-no-nuclear-license-2014': {'id': 'BSD-3-Clause-No-Nuclear-License-2014'},
+ 'bsd-3-clause-no-nuclear-warranty': {'id': 'BSD-3-Clause-No-Nuclear-Warranty'},
+ 'bsd-3-clause-open-mpi': {'id': 'BSD-3-Clause-Open-MPI'},
+ 'bsd-3-clause-sun': {'id': 'BSD-3-Clause-Sun'},
+ 'bsd-4-clause': {'id': 'BSD-4-Clause'},
+ 'bsd-4-clause-shortened': {'id': 'BSD-4-Clause-Shortened'},
+ 'bsd-4-clause-uc': {'id': 'BSD-4-Clause-UC'},
+ 'bsd-4.3reno': {'id': 'BSD-4.3RENO'},
+ 'bsd-4.3tahoe': {'id': 'BSD-4.3TAHOE'},
+ 'bsd-advertising-acknowledgement': {'id': 'BSD-Advertising-Acknowledgement'},
+ 'bsd-attribution-hpnd-disclaimer': {'id': 'BSD-Attribution-HPND-disclaimer'},
+ 'bsd-inferno-nettverk': {'id': 'BSD-Inferno-Nettverk'},
+ 'bsd-protection': {'id': 'BSD-Protection'},
+ 'bsd-source-beginning-file': {'id': 'BSD-Source-beginning-file'},
+ 'bsd-source-code': {'id': 'BSD-Source-Code'},
+ 'bsd-systemics': {'id': 'BSD-Systemics'},
+ 'bsd-systemics-w3works': {'id': 'BSD-Systemics-W3Works'},
+ 'bsl-1.0': {'id': 'BSL-1.0'},
+ 'busl-1.1': {'id': 'BUSL-1.1'},
+ 'bzip2-1.0.6': {'id': 'bzip2-1.0.6'},
+ 'c-uda-1.0': {'id': 'C-UDA-1.0'},
+ 'cal-1.0': {'id': 'CAL-1.0'},
+ 'cal-1.0-combined-work-exception': {'id': 'CAL-1.0-Combined-Work-Exception'},
+ 'caldera': {'id': 'Caldera'},
+ 'caldera-no-preamble': {'id': 'Caldera-no-preamble'},
+ 'catharon': {'id': 'Catharon'},
+ 'catosl-1.1': {'id': 'CATOSL-1.1'},
+ 'cc-by-1.0': {'id': 'CC-BY-1.0'},
+ 'cc-by-2.0': {'id': 'CC-BY-2.0'},
+ 'cc-by-2.5': {'id': 'CC-BY-2.5'},
+ 'cc-by-2.5-au': {'id': 'CC-BY-2.5-AU'},
+ 'cc-by-3.0': {'id': 'CC-BY-3.0'},
+ 'cc-by-3.0-at': {'id': 'CC-BY-3.0-AT'},
+ 'cc-by-3.0-au': {'id': 'CC-BY-3.0-AU'},
+ 'cc-by-3.0-de': {'id': 'CC-BY-3.0-DE'},
+ 'cc-by-3.0-igo': {'id': 'CC-BY-3.0-IGO'},
+ 'cc-by-3.0-nl': {'id': 'CC-BY-3.0-NL'},
+ 'cc-by-3.0-us': {'id': 'CC-BY-3.0-US'},
+ 'cc-by-4.0': {'id': 'CC-BY-4.0'},
+ 'cc-by-nc-1.0': {'id': 'CC-BY-NC-1.0'},
+ 'cc-by-nc-2.0': {'id': 'CC-BY-NC-2.0'},
+ 'cc-by-nc-2.5': {'id': 'CC-BY-NC-2.5'},
+ 'cc-by-nc-3.0': {'id': 'CC-BY-NC-3.0'},
+ 'cc-by-nc-3.0-de': {'id': 'CC-BY-NC-3.0-DE'},
+ 'cc-by-nc-4.0': {'id': 'CC-BY-NC-4.0'},
+ 'cc-by-nc-nd-1.0': {'id': 'CC-BY-NC-ND-1.0'},
+ 'cc-by-nc-nd-2.0': {'id': 'CC-BY-NC-ND-2.0'},
+ 'cc-by-nc-nd-2.5': {'id': 'CC-BY-NC-ND-2.5'},
+ 'cc-by-nc-nd-3.0': {'id': 'CC-BY-NC-ND-3.0'},
+ 'cc-by-nc-nd-3.0-de': {'id': 'CC-BY-NC-ND-3.0-DE'},
+ 'cc-by-nc-nd-3.0-igo': {'id': 'CC-BY-NC-ND-3.0-IGO'},
+ 'cc-by-nc-nd-4.0': {'id': 'CC-BY-NC-ND-4.0'},
+ 'cc-by-nc-sa-1.0': {'id': 'CC-BY-NC-SA-1.0'},
+ 'cc-by-nc-sa-2.0': {'id': 'CC-BY-NC-SA-2.0'},
+ 'cc-by-nc-sa-2.0-de': {'id': 'CC-BY-NC-SA-2.0-DE'},
+ 'cc-by-nc-sa-2.0-fr': {'id': 'CC-BY-NC-SA-2.0-FR'},
+ 'cc-by-nc-sa-2.0-uk': {'id': 'CC-BY-NC-SA-2.0-UK'},
+ 'cc-by-nc-sa-2.5': {'id': 'CC-BY-NC-SA-2.5'},
+ 'cc-by-nc-sa-3.0': {'id': 'CC-BY-NC-SA-3.0'},
+ 'cc-by-nc-sa-3.0-de': {'id': 'CC-BY-NC-SA-3.0-DE'},
+ 'cc-by-nc-sa-3.0-igo': {'id': 'CC-BY-NC-SA-3.0-IGO'},
+ 'cc-by-nc-sa-4.0': {'id': 'CC-BY-NC-SA-4.0'},
+ 'cc-by-nd-1.0': {'id': 'CC-BY-ND-1.0'},
+ 'cc-by-nd-2.0': {'id': 'CC-BY-ND-2.0'},
+ 'cc-by-nd-2.5': {'id': 'CC-BY-ND-2.5'},
+ 'cc-by-nd-3.0': {'id': 'CC-BY-ND-3.0'},
+ 'cc-by-nd-3.0-de': {'id': 'CC-BY-ND-3.0-DE'},
+ 'cc-by-nd-4.0': {'id': 'CC-BY-ND-4.0'},
+ 'cc-by-sa-1.0': {'id': 'CC-BY-SA-1.0'},
+ 'cc-by-sa-2.0': {'id': 'CC-BY-SA-2.0'},
+ 'cc-by-sa-2.0-uk': {'id': 'CC-BY-SA-2.0-UK'},
+ 'cc-by-sa-2.1-jp': {'id': 'CC-BY-SA-2.1-JP'},
+ 'cc-by-sa-2.5': {'id': 'CC-BY-SA-2.5'},
+ 'cc-by-sa-3.0': {'id': 'CC-BY-SA-3.0'},
+ 'cc-by-sa-3.0-at': {'id': 'CC-BY-SA-3.0-AT'},
+ 'cc-by-sa-3.0-de': {'id': 'CC-BY-SA-3.0-DE'},
+ 'cc-by-sa-3.0-igo': {'id': 'CC-BY-SA-3.0-IGO'},
+ 'cc-by-sa-4.0': {'id': 'CC-BY-SA-4.0'},
+ 'cc-pddc': {'id': 'CC-PDDC'},
+ 'cc-pdm-1.0': {'id': 'CC-PDM-1.0'},
+ 'cc-sa-1.0': {'id': 'CC-SA-1.0'},
+ 'cc0-1.0': {'id': 'CC0-1.0'},
+ 'cddl-1.0': {'id': 'CDDL-1.0'},
+ 'cddl-1.1': {'id': 'CDDL-1.1'},
+ 'cdl-1.0': {'id': 'CDL-1.0'},
+ 'cdla-permissive-1.0': {'id': 'CDLA-Permissive-1.0'},
+ 'cdla-permissive-2.0': {'id': 'CDLA-Permissive-2.0'},
+ 'cdla-sharing-1.0': {'id': 'CDLA-Sharing-1.0'},
+ 'cecill-1.0': {'id': 'CECILL-1.0'},
+ 'cecill-1.1': {'id': 'CECILL-1.1'},
+ 'cecill-2.0': {'id': 'CECILL-2.0'},
+ 'cecill-2.1': {'id': 'CECILL-2.1'},
+ 'cecill-b': {'id': 'CECILL-B'},
+ 'cecill-c': {'id': 'CECILL-C'},
+ 'cern-ohl-1.1': {'id': 'CERN-OHL-1.1'},
+ 'cern-ohl-1.2': {'id': 'CERN-OHL-1.2'},
+ 'cern-ohl-p-2.0': {'id': 'CERN-OHL-P-2.0'},
+ 'cern-ohl-s-2.0': {'id': 'CERN-OHL-S-2.0'},
+ 'cern-ohl-w-2.0': {'id': 'CERN-OHL-W-2.0'},
+ 'cfitsio': {'id': 'CFITSIO'},
+ 'check-cvs': {'id': 'check-cvs'},
+ 'checkmk': {'id': 'checkmk'},
+ 'clartistic': {'id': 'ClArtistic'},
+ 'clips': {'id': 'Clips'},
+ 'cmu-mach': {'id': 'CMU-Mach'},
+ 'cmu-mach-nodoc': {'id': 'CMU-Mach-nodoc'},
+ 'cnri-jython': {'id': 'CNRI-Jython'},
+ 'cnri-python': {'id': 'CNRI-Python'},
+ 'cnri-python-gpl-compatible': {'id': 'CNRI-Python-GPL-Compatible'},
+ 'coil-1.0': {'id': 'COIL-1.0'},
+ 'community-spec-1.0': {'id': 'Community-Spec-1.0'},
+ 'condor-1.1': {'id': 'Condor-1.1'},
+ 'copyleft-next-0.3.0': {'id': 'copyleft-next-0.3.0'},
+ 'copyleft-next-0.3.1': {'id': 'copyleft-next-0.3.1'},
+ 'cornell-lossless-jpeg': {'id': 'Cornell-Lossless-JPEG'},
+ 'cpal-1.0': {'id': 'CPAL-1.0'},
+ 'cpl-1.0': {'id': 'CPL-1.0'},
+ 'cpol-1.02': {'id': 'CPOL-1.02'},
+ 'cronyx': {'id': 'Cronyx'},
+ 'crossword': {'id': 'Crossword'},
+ 'crystalstacker': {'id': 'CrystalStacker'},
+ 'cua-opl-1.0': {'id': 'CUA-OPL-1.0'},
+ 'cube': {'id': 'Cube'},
+ 'curl': {'id': 'curl'},
+ 'cve-tou': {'id': 'cve-tou'},
+ 'd-fsl-1.0': {'id': 'D-FSL-1.0'},
+ 'dec-3-clause': {'id': 'DEC-3-Clause'},
+ 'diffmark': {'id': 'diffmark'},
+ 'dl-de-by-2.0': {'id': 'DL-DE-BY-2.0'},
+ 'dl-de-zero-2.0': {'id': 'DL-DE-ZERO-2.0'},
+ 'doc': {'id': 'DOC'},
+ 'docbook-schema': {'id': 'DocBook-Schema'},
+ 'docbook-stylesheet': {'id': 'DocBook-Stylesheet'},
+ 'docbook-xml': {'id': 'DocBook-XML'},
+ 'dotseqn': {'id': 'Dotseqn'},
+ 'drl-1.0': {'id': 'DRL-1.0'},
+ 'drl-1.1': {'id': 'DRL-1.1'},
+ 'dsdp': {'id': 'DSDP'},
+ 'dtoa': {'id': 'dtoa'},
+ 'dvipdfm': {'id': 'dvipdfm'},
+ 'ecl-1.0': {'id': 'ECL-1.0'},
+ 'ecl-2.0': {'id': 'ECL-2.0'},
+ 'efl-1.0': {'id': 'EFL-1.0'},
+ 'efl-2.0': {'id': 'EFL-2.0'},
+ 'egenix': {'id': 'eGenix'},
+ 'elastic-2.0': {'id': 'Elastic-2.0'},
+ 'entessa': {'id': 'Entessa'},
+ 'epics': {'id': 'EPICS'},
+ 'epl-1.0': {'id': 'EPL-1.0'},
+ 'epl-2.0': {'id': 'EPL-2.0'},
+ 'erlpl-1.1': {'id': 'ErlPL-1.1'},
+ 'etalab-2.0': {'id': 'etalab-2.0'},
+ 'eudatagrid': {'id': 'EUDatagrid'},
+ 'eupl-1.0': {'id': 'EUPL-1.0'},
+ 'eupl-1.1': {'id': 'EUPL-1.1'},
+ 'eupl-1.2': {'id': 'EUPL-1.2'},
+ 'eurosym': {'id': 'Eurosym'},
+ 'fair': {'id': 'Fair'},
+ 'fbm': {'id': 'FBM'},
+ 'fdk-aac': {'id': 'FDK-AAC'},
+ 'ferguson-twofish': {'id': 'Ferguson-Twofish'},
+ 'frameworx-1.0': {'id': 'Frameworx-1.0'},
+ 'freebsd-doc': {'id': 'FreeBSD-DOC'},
+ 'freeimage': {'id': 'FreeImage'},
+ 'fsfap': {'id': 'FSFAP'},
+ 'fsfap-no-warranty-disclaimer': {'id': 'FSFAP-no-warranty-disclaimer'},
+ 'fsful': {'id': 'FSFUL'},
+ 'fsfullr': {'id': 'FSFULLR'},
+ 'fsfullrwd': {'id': 'FSFULLRWD'},
+ 'ftl': {'id': 'FTL'},
+ 'furuseth': {'id': 'Furuseth'},
+ 'fwlw': {'id': 'fwlw'},
+ 'gcr-docs': {'id': 'GCR-docs'},
+ 'gd': {'id': 'GD'},
+ 'generic-xts': {'id': 'generic-xts'},
+ 'gfdl-1.1-invariants-only': {'id': 'GFDL-1.1-invariants-only'},
+ 'gfdl-1.1-invariants-or-later': {'id': 'GFDL-1.1-invariants-or-later'},
+ 'gfdl-1.1-no-invariants-only': {'id': 'GFDL-1.1-no-invariants-only'},
+ 'gfdl-1.1-no-invariants-or-later': {'id': 'GFDL-1.1-no-invariants-or-later'},
+ 'gfdl-1.1-only': {'id': 'GFDL-1.1-only'},
+ 'gfdl-1.1-or-later': {'id': 'GFDL-1.1-or-later'},
+ 'gfdl-1.2-invariants-only': {'id': 'GFDL-1.2-invariants-only'},
+ 'gfdl-1.2-invariants-or-later': {'id': 'GFDL-1.2-invariants-or-later'},
+ 'gfdl-1.2-no-invariants-only': {'id': 'GFDL-1.2-no-invariants-only'},
+ 'gfdl-1.2-no-invariants-or-later': {'id': 'GFDL-1.2-no-invariants-or-later'},
+ 'gfdl-1.2-only': {'id': 'GFDL-1.2-only'},
+ 'gfdl-1.2-or-later': {'id': 'GFDL-1.2-or-later'},
+ 'gfdl-1.3-invariants-only': {'id': 'GFDL-1.3-invariants-only'},
+ 'gfdl-1.3-invariants-or-later': {'id': 'GFDL-1.3-invariants-or-later'},
+ 'gfdl-1.3-no-invariants-only': {'id': 'GFDL-1.3-no-invariants-only'},
+ 'gfdl-1.3-no-invariants-or-later': {'id': 'GFDL-1.3-no-invariants-or-later'},
+ 'gfdl-1.3-only': {'id': 'GFDL-1.3-only'},
+ 'gfdl-1.3-or-later': {'id': 'GFDL-1.3-or-later'},
+ 'giftware': {'id': 'Giftware'},
+ 'gl2ps': {'id': 'GL2PS'},
+ 'glide': {'id': 'Glide'},
+ 'glulxe': {'id': 'Glulxe'},
+ 'glwtpl': {'id': 'GLWTPL'},
+ 'gnuplot': {'id': 'gnuplot'},
+ 'gpl-1.0-only': {'id': 'GPL-1.0-only'},
+ 'gpl-1.0-or-later': {'id': 'GPL-1.0-or-later'},
+ 'gpl-2.0-only': {'id': 'GPL-2.0-only'},
+ 'gpl-2.0-or-later': {'id': 'GPL-2.0-or-later'},
+ 'gpl-3.0-only': {'id': 'GPL-3.0-only'},
+ 'gpl-3.0-or-later': {'id': 'GPL-3.0-or-later'},
+ 'graphics-gems': {'id': 'Graphics-Gems'},
+ 'gsoap-1.3b': {'id': 'gSOAP-1.3b'},
+ 'gtkbook': {'id': 'gtkbook'},
+ 'gutmann': {'id': 'Gutmann'},
+ 'haskellreport': {'id': 'HaskellReport'},
+ 'hdparm': {'id': 'hdparm'},
+ 'hidapi': {'id': 'HIDAPI'},
+ 'hippocratic-2.1': {'id': 'Hippocratic-2.1'},
+ 'hp-1986': {'id': 'HP-1986'},
+ 'hp-1989': {'id': 'HP-1989'},
+ 'hpnd': {'id': 'HPND'},
+ 'hpnd-dec': {'id': 'HPND-DEC'},
+ 'hpnd-doc': {'id': 'HPND-doc'},
+ 'hpnd-doc-sell': {'id': 'HPND-doc-sell'},
+ 'hpnd-export-us': {'id': 'HPND-export-US'},
+ 'hpnd-export-us-acknowledgement': {'id': 'HPND-export-US-acknowledgement'},
+ 'hpnd-export-us-modify': {'id': 'HPND-export-US-modify'},
+ 'hpnd-export2-us': {'id': 'HPND-export2-US'},
+ 'hpnd-fenneberg-livingston': {'id': 'HPND-Fenneberg-Livingston'},
+ 'hpnd-inria-imag': {'id': 'HPND-INRIA-IMAG'},
+ 'hpnd-intel': {'id': 'HPND-Intel'},
+ 'hpnd-kevlin-henney': {'id': 'HPND-Kevlin-Henney'},
+ 'hpnd-markus-kuhn': {'id': 'HPND-Markus-Kuhn'},
+ 'hpnd-merchantability-variant': {'id': 'HPND-merchantability-variant'},
+ 'hpnd-mit-disclaimer': {'id': 'HPND-MIT-disclaimer'},
+ 'hpnd-netrek': {'id': 'HPND-Netrek'},
+ 'hpnd-pbmplus': {'id': 'HPND-Pbmplus'},
+ 'hpnd-sell-mit-disclaimer-xserver': {'id': 'HPND-sell-MIT-disclaimer-xserver'},
+ 'hpnd-sell-regexpr': {'id': 'HPND-sell-regexpr'},
+ 'hpnd-sell-variant': {'id': 'HPND-sell-variant'},
+ 'hpnd-sell-variant-mit-disclaimer': {'id': 'HPND-sell-variant-MIT-disclaimer'},
+ 'hpnd-sell-variant-mit-disclaimer-rev': {'id': 'HPND-sell-variant-MIT-disclaimer-rev'},
+ 'hpnd-uc': {'id': 'HPND-UC'},
+ 'hpnd-uc-export-us': {'id': 'HPND-UC-export-US'},
+ 'htmltidy': {'id': 'HTMLTIDY'},
+ 'ibm-pibs': {'id': 'IBM-pibs'},
+ 'icu': {'id': 'ICU'},
+ 'iec-code-components-eula': {'id': 'IEC-Code-Components-EULA'},
+ 'ijg': {'id': 'IJG'},
+ 'ijg-short': {'id': 'IJG-short'},
+ 'imagemagick': {'id': 'ImageMagick'},
+ 'imatix': {'id': 'iMatix'},
+ 'imlib2': {'id': 'Imlib2'},
+ 'info-zip': {'id': 'Info-ZIP'},
+ 'inner-net-2.0': {'id': 'Inner-Net-2.0'},
+ 'innosetup': {'id': 'InnoSetup'},
+ 'intel': {'id': 'Intel'},
+ 'intel-acpi': {'id': 'Intel-ACPI'},
+ 'interbase-1.0': {'id': 'Interbase-1.0'},
+ 'ipa': {'id': 'IPA'},
+ 'ipl-1.0': {'id': 'IPL-1.0'},
+ 'isc': {'id': 'ISC'},
+ 'isc-veillard': {'id': 'ISC-Veillard'},
+ 'jam': {'id': 'Jam'},
+ 'jasper-2.0': {'id': 'JasPer-2.0'},
+ 'jpl-image': {'id': 'JPL-image'},
+ 'jpnic': {'id': 'JPNIC'},
+ 'json': {'id': 'JSON'},
+ 'kastrup': {'id': 'Kastrup'},
+ 'kazlib': {'id': 'Kazlib'},
+ 'knuth-ctan': {'id': 'Knuth-CTAN'},
+ 'lal-1.2': {'id': 'LAL-1.2'},
+ 'lal-1.3': {'id': 'LAL-1.3'},
+ 'latex2e': {'id': 'Latex2e'},
+ 'latex2e-translated-notice': {'id': 'Latex2e-translated-notice'},
+ 'leptonica': {'id': 'Leptonica'},
+ 'lgpl-2.0-only': {'id': 'LGPL-2.0-only'},
+ 'lgpl-2.0-or-later': {'id': 'LGPL-2.0-or-later'},
+ 'lgpl-2.1-only': {'id': 'LGPL-2.1-only'},
+ 'lgpl-2.1-or-later': {'id': 'LGPL-2.1-or-later'},
+ 'lgpl-3.0-only': {'id': 'LGPL-3.0-only'},
+ 'lgpl-3.0-or-later': {'id': 'LGPL-3.0-or-later'},
+ 'lgpllr': {'id': 'LGPLLR'},
+ 'libpng': {'id': 'Libpng'},
+ 'libpng-2.0': {'id': 'libpng-2.0'},
+ 'libselinux-1.0': {'id': 'libselinux-1.0'},
+ 'libtiff': {'id': 'libtiff'},
+ 'libutil-david-nugent': {'id': 'libutil-David-Nugent'},
+ 'liliq-p-1.1': {'id': 'LiLiQ-P-1.1'},
+ 'liliq-r-1.1': {'id': 'LiLiQ-R-1.1'},
+ 'liliq-rplus-1.1': {'id': 'LiLiQ-Rplus-1.1'},
+ 'linux-man-pages-1-para': {'id': 'Linux-man-pages-1-para'},
+ 'linux-man-pages-copyleft': {'id': 'Linux-man-pages-copyleft'},
+ 'linux-man-pages-copyleft-2-para': {'id': 'Linux-man-pages-copyleft-2-para'},
+ 'linux-man-pages-copyleft-var': {'id': 'Linux-man-pages-copyleft-var'},
+ 'linux-openib': {'id': 'Linux-OpenIB'},
+ 'loop': {'id': 'LOOP'},
+ 'lpd-document': {'id': 'LPD-document'},
+ 'lpl-1.0': {'id': 'LPL-1.0'},
+ 'lpl-1.02': {'id': 'LPL-1.02'},
+ 'lppl-1.0': {'id': 'LPPL-1.0'},
+ 'lppl-1.1': {'id': 'LPPL-1.1'},
+ 'lppl-1.2': {'id': 'LPPL-1.2'},
+ 'lppl-1.3a': {'id': 'LPPL-1.3a'},
+ 'lppl-1.3c': {'id': 'LPPL-1.3c'},
+ 'lsof': {'id': 'lsof'},
+ 'lucida-bitmap-fonts': {'id': 'Lucida-Bitmap-Fonts'},
+ 'lzma-sdk-9.11-to-9.20': {'id': 'LZMA-SDK-9.11-to-9.20'},
+ 'lzma-sdk-9.22': {'id': 'LZMA-SDK-9.22'},
+ 'mackerras-3-clause': {'id': 'Mackerras-3-Clause'},
+ 'mackerras-3-clause-acknowledgment': {'id': 'Mackerras-3-Clause-acknowledgment'},
+ 'magaz': {'id': 'magaz'},
+ 'mailprio': {'id': 'mailprio'},
+ 'makeindex': {'id': 'MakeIndex'},
+ 'martin-birgmeier': {'id': 'Martin-Birgmeier'},
+ 'mcphee-slideshow': {'id': 'McPhee-slideshow'},
+ 'metamail': {'id': 'metamail'},
+ 'minpack': {'id': 'Minpack'},
+ 'mips': {'id': 'MIPS'},
+ 'miros': {'id': 'MirOS'},
+ 'mit': {'id': 'MIT'},
+ 'mit-0': {'id': 'MIT-0'},
+ 'mit-advertising': {'id': 'MIT-advertising'},
+ 'mit-click': {'id': 'MIT-Click'},
+ 'mit-cmu': {'id': 'MIT-CMU'},
+ 'mit-enna': {'id': 'MIT-enna'},
+ 'mit-feh': {'id': 'MIT-feh'},
+ 'mit-festival': {'id': 'MIT-Festival'},
+ 'mit-khronos-old': {'id': 'MIT-Khronos-old'},
+ 'mit-modern-variant': {'id': 'MIT-Modern-Variant'},
+ 'mit-open-group': {'id': 'MIT-open-group'},
+ 'mit-testregex': {'id': 'MIT-testregex'},
+ 'mit-wu': {'id': 'MIT-Wu'},
+ 'mitnfa': {'id': 'MITNFA'},
+ 'mmixware': {'id': 'MMIXware'},
+ 'motosoto': {'id': 'Motosoto'},
+ 'mpeg-ssg': {'id': 'MPEG-SSG'},
+ 'mpi-permissive': {'id': 'mpi-permissive'},
+ 'mpich2': {'id': 'mpich2'},
+ 'mpl-1.0': {'id': 'MPL-1.0'},
+ 'mpl-1.1': {'id': 'MPL-1.1'},
+ 'mpl-2.0': {'id': 'MPL-2.0'},
+ 'mpl-2.0-no-copyleft-exception': {'id': 'MPL-2.0-no-copyleft-exception'},
+ 'mplus': {'id': 'mplus'},
+ 'ms-lpl': {'id': 'MS-LPL'},
+ 'ms-pl': {'id': 'MS-PL'},
+ 'ms-rl': {'id': 'MS-RL'},
+ 'mtll': {'id': 'MTLL'},
+ 'mulanpsl-1.0': {'id': 'MulanPSL-1.0'},
+ 'mulanpsl-2.0': {'id': 'MulanPSL-2.0'},
+ 'multics': {'id': 'Multics'},
+ 'mup': {'id': 'Mup'},
+ 'naist-2003': {'id': 'NAIST-2003'},
+ 'nasa-1.3': {'id': 'NASA-1.3'},
+ 'naumen': {'id': 'Naumen'},
+ 'nbpl-1.0': {'id': 'NBPL-1.0'},
+ 'ncbi-pd': {'id': 'NCBI-PD'},
+ 'ncgl-uk-2.0': {'id': 'NCGL-UK-2.0'},
+ 'ncl': {'id': 'NCL'},
+ 'ncsa': {'id': 'NCSA'},
+ 'netcdf': {'id': 'NetCDF'},
+ 'newsletr': {'id': 'Newsletr'},
+ 'ngpl': {'id': 'NGPL'},
+ 'nicta-1.0': {'id': 'NICTA-1.0'},
+ 'nist-pd': {'id': 'NIST-PD'},
+ 'nist-pd-fallback': {'id': 'NIST-PD-fallback'},
+ 'nist-software': {'id': 'NIST-Software'},
+ 'nlod-1.0': {'id': 'NLOD-1.0'},
+ 'nlod-2.0': {'id': 'NLOD-2.0'},
+ 'nlpl': {'id': 'NLPL'},
+ 'nokia': {'id': 'Nokia'},
+ 'nosl': {'id': 'NOSL'},
+ 'noweb': {'id': 'Noweb'},
+ 'npl-1.0': {'id': 'NPL-1.0'},
+ 'npl-1.1': {'id': 'NPL-1.1'},
+ 'nposl-3.0': {'id': 'NPOSL-3.0'},
+ 'nrl': {'id': 'NRL'},
+ 'ntp': {'id': 'NTP'},
+ 'ntp-0': {'id': 'NTP-0'},
+ 'o-uda-1.0': {'id': 'O-UDA-1.0'},
+ 'oar': {'id': 'OAR'},
+ 'occt-pl': {'id': 'OCCT-PL'},
+ 'oclc-2.0': {'id': 'OCLC-2.0'},
+ 'odbl-1.0': {'id': 'ODbL-1.0'},
+ 'odc-by-1.0': {'id': 'ODC-By-1.0'},
+ 'offis': {'id': 'OFFIS'},
+ 'ofl-1.0': {'id': 'OFL-1.0'},
+ 'ofl-1.0-no-rfn': {'id': 'OFL-1.0-no-RFN'},
+ 'ofl-1.0-rfn': {'id': 'OFL-1.0-RFN'},
+ 'ofl-1.1': {'id': 'OFL-1.1'},
+ 'ofl-1.1-no-rfn': {'id': 'OFL-1.1-no-RFN'},
+ 'ofl-1.1-rfn': {'id': 'OFL-1.1-RFN'},
+ 'ogc-1.0': {'id': 'OGC-1.0'},
+ 'ogdl-taiwan-1.0': {'id': 'OGDL-Taiwan-1.0'},
+ 'ogl-canada-2.0': {'id': 'OGL-Canada-2.0'},
+ 'ogl-uk-1.0': {'id': 'OGL-UK-1.0'},
+ 'ogl-uk-2.0': {'id': 'OGL-UK-2.0'},
+ 'ogl-uk-3.0': {'id': 'OGL-UK-3.0'},
+ 'ogtsl': {'id': 'OGTSL'},
+ 'oldap-1.1': {'id': 'OLDAP-1.1'},
+ 'oldap-1.2': {'id': 'OLDAP-1.2'},
+ 'oldap-1.3': {'id': 'OLDAP-1.3'},
+ 'oldap-1.4': {'id': 'OLDAP-1.4'},
+ 'oldap-2.0': {'id': 'OLDAP-2.0'},
+ 'oldap-2.0.1': {'id': 'OLDAP-2.0.1'},
+ 'oldap-2.1': {'id': 'OLDAP-2.1'},
+ 'oldap-2.2': {'id': 'OLDAP-2.2'},
+ 'oldap-2.2.1': {'id': 'OLDAP-2.2.1'},
+ 'oldap-2.2.2': {'id': 'OLDAP-2.2.2'},
+ 'oldap-2.3': {'id': 'OLDAP-2.3'},
+ 'oldap-2.4': {'id': 'OLDAP-2.4'},
+ 'oldap-2.5': {'id': 'OLDAP-2.5'},
+ 'oldap-2.6': {'id': 'OLDAP-2.6'},
+ 'oldap-2.7': {'id': 'OLDAP-2.7'},
+ 'oldap-2.8': {'id': 'OLDAP-2.8'},
+ 'olfl-1.3': {'id': 'OLFL-1.3'},
+ 'oml': {'id': 'OML'},
+ 'openpbs-2.3': {'id': 'OpenPBS-2.3'},
+ 'openssl': {'id': 'OpenSSL'},
+ 'openssl-standalone': {'id': 'OpenSSL-standalone'},
+ 'openvision': {'id': 'OpenVision'},
+ 'opl-1.0': {'id': 'OPL-1.0'},
+ 'opl-uk-3.0': {'id': 'OPL-UK-3.0'},
+ 'opubl-1.0': {'id': 'OPUBL-1.0'},
+ 'oset-pl-2.1': {'id': 'OSET-PL-2.1'},
+ 'osl-1.0': {'id': 'OSL-1.0'},
+ 'osl-1.1': {'id': 'OSL-1.1'},
+ 'osl-2.0': {'id': 'OSL-2.0'},
+ 'osl-2.1': {'id': 'OSL-2.1'},
+ 'osl-3.0': {'id': 'OSL-3.0'},
+ 'padl': {'id': 'PADL'},
+ 'parity-6.0.0': {'id': 'Parity-6.0.0'},
+ 'parity-7.0.0': {'id': 'Parity-7.0.0'},
+ 'pddl-1.0': {'id': 'PDDL-1.0'},
+ 'php-3.0': {'id': 'PHP-3.0'},
+ 'php-3.01': {'id': 'PHP-3.01'},
+ 'pixar': {'id': 'Pixar'},
+ 'pkgconf': {'id': 'pkgconf'},
+ 'plexus': {'id': 'Plexus'},
+ 'pnmstitch': {'id': 'pnmstitch'},
+ 'polyform-noncommercial-1.0.0': {'id': 'PolyForm-Noncommercial-1.0.0'},
+ 'polyform-small-business-1.0.0': {'id': 'PolyForm-Small-Business-1.0.0'},
+ 'postgresql': {'id': 'PostgreSQL'},
+ 'ppl': {'id': 'PPL'},
+ 'psf-2.0': {'id': 'PSF-2.0'},
+ 'psfrag': {'id': 'psfrag'},
+ 'psutils': {'id': 'psutils'},
+ 'python-2.0': {'id': 'Python-2.0'},
+ 'python-2.0.1': {'id': 'Python-2.0.1'},
+ 'python-ldap': {'id': 'python-ldap'},
+ 'qhull': {'id': 'Qhull'},
+ 'qpl-1.0': {'id': 'QPL-1.0'},
+ 'qpl-1.0-inria-2004': {'id': 'QPL-1.0-INRIA-2004'},
+ 'radvd': {'id': 'radvd'},
+ 'rdisc': {'id': 'Rdisc'},
+ 'rhecos-1.1': {'id': 'RHeCos-1.1'},
+ 'rpl-1.1': {'id': 'RPL-1.1'},
+ 'rpl-1.5': {'id': 'RPL-1.5'},
+ 'rpsl-1.0': {'id': 'RPSL-1.0'},
+ 'rsa-md': {'id': 'RSA-MD'},
+ 'rscpl': {'id': 'RSCPL'},
+ 'ruby': {'id': 'Ruby'},
+ 'ruby-pty': {'id': 'Ruby-pty'},
+ 'sax-pd': {'id': 'SAX-PD'},
+ 'sax-pd-2.0': {'id': 'SAX-PD-2.0'},
+ 'saxpath': {'id': 'Saxpath'},
+ 'scea': {'id': 'SCEA'},
+ 'schemereport': {'id': 'SchemeReport'},
+ 'sendmail': {'id': 'Sendmail'},
+ 'sendmail-8.23': {'id': 'Sendmail-8.23'},
+ 'sendmail-open-source-1.1': {'id': 'Sendmail-Open-Source-1.1'},
+ 'sgi-b-1.0': {'id': 'SGI-B-1.0'},
+ 'sgi-b-1.1': {'id': 'SGI-B-1.1'},
+ 'sgi-b-2.0': {'id': 'SGI-B-2.0'},
+ 'sgi-opengl': {'id': 'SGI-OpenGL'},
+ 'sgp4': {'id': 'SGP4'},
+ 'shl-0.5': {'id': 'SHL-0.5'},
+ 'shl-0.51': {'id': 'SHL-0.51'},
+ 'simpl-2.0': {'id': 'SimPL-2.0'},
+ 'sissl': {'id': 'SISSL'},
+ 'sissl-1.2': {'id': 'SISSL-1.2'},
+ 'sl': {'id': 'SL'},
+ 'sleepycat': {'id': 'Sleepycat'},
+ 'smail-gpl': {'id': 'SMAIL-GPL'},
+ 'smlnj': {'id': 'SMLNJ'},
+ 'smppl': {'id': 'SMPPL'},
+ 'snia': {'id': 'SNIA'},
+ 'snprintf': {'id': 'snprintf'},
+ 'softsurfer': {'id': 'softSurfer'},
+ 'soundex': {'id': 'Soundex'},
+ 'spencer-86': {'id': 'Spencer-86'},
+ 'spencer-94': {'id': 'Spencer-94'},
+ 'spencer-99': {'id': 'Spencer-99'},
+ 'spl-1.0': {'id': 'SPL-1.0'},
+ 'ssh-keyscan': {'id': 'ssh-keyscan'},
+ 'ssh-openssh': {'id': 'SSH-OpenSSH'},
+ 'ssh-short': {'id': 'SSH-short'},
+ 'ssleay-standalone': {'id': 'SSLeay-standalone'},
+ 'sspl-1.0': {'id': 'SSPL-1.0'},
+ 'sugarcrm-1.1.3': {'id': 'SugarCRM-1.1.3'},
+ 'sun-ppp': {'id': 'Sun-PPP'},
+ 'sun-ppp-2000': {'id': 'Sun-PPP-2000'},
+ 'sunpro': {'id': 'SunPro'},
+ 'swl': {'id': 'SWL'},
+ 'swrule': {'id': 'swrule'},
+ 'symlinks': {'id': 'Symlinks'},
+ 'tapr-ohl-1.0': {'id': 'TAPR-OHL-1.0'},
+ 'tcl': {'id': 'TCL'},
+ 'tcp-wrappers': {'id': 'TCP-wrappers'},
+ 'termreadkey': {'id': 'TermReadKey'},
+ 'tgppl-1.0': {'id': 'TGPPL-1.0'},
+ 'thirdeye': {'id': 'ThirdEye'},
+ 'threeparttable': {'id': 'threeparttable'},
+ 'tmate': {'id': 'TMate'},
+ 'torque-1.1': {'id': 'TORQUE-1.1'},
+ 'tosl': {'id': 'TOSL'},
+ 'tpdl': {'id': 'TPDL'},
+ 'tpl-1.0': {'id': 'TPL-1.0'},
+ 'trustedqsl': {'id': 'TrustedQSL'},
+ 'ttwl': {'id': 'TTWL'},
+ 'ttyp0': {'id': 'TTYP0'},
+ 'tu-berlin-1.0': {'id': 'TU-Berlin-1.0'},
+ 'tu-berlin-2.0': {'id': 'TU-Berlin-2.0'},
+ 'ubuntu-font-1.0': {'id': 'Ubuntu-font-1.0'},
+ 'ucar': {'id': 'UCAR'},
+ 'ucl-1.0': {'id': 'UCL-1.0'},
+ 'ulem': {'id': 'ulem'},
+ 'umich-merit': {'id': 'UMich-Merit'},
+ 'unicode-3.0': {'id': 'Unicode-3.0'},
+ 'unicode-dfs-2015': {'id': 'Unicode-DFS-2015'},
+ 'unicode-dfs-2016': {'id': 'Unicode-DFS-2016'},
+ 'unicode-tou': {'id': 'Unicode-TOU'},
+ 'unixcrypt': {'id': 'UnixCrypt'},
+ 'unlicense': {'id': 'Unlicense'},
+ 'upl-1.0': {'id': 'UPL-1.0'},
+ 'urt-rle': {'id': 'URT-RLE'},
+ 'vim': {'id': 'Vim'},
+ 'vostrom': {'id': 'VOSTROM'},
+ 'vsl-1.0': {'id': 'VSL-1.0'},
+ 'w3c': {'id': 'W3C'},
+ 'w3c-19980720': {'id': 'W3C-19980720'},
+ 'w3c-20150513': {'id': 'W3C-20150513'},
+ 'w3m': {'id': 'w3m'},
+ 'watcom-1.0': {'id': 'Watcom-1.0'},
+ 'widget-workshop': {'id': 'Widget-Workshop'},
+ 'wsuipa': {'id': 'Wsuipa'},
+ 'wtfpl': {'id': 'WTFPL'},
+ 'wwl': {'id': 'wwl'},
+ 'x11': {'id': 'X11'},
+ 'x11-distribute-modifications-variant': {'id': 'X11-distribute-modifications-variant'},
+ 'x11-swapped': {'id': 'X11-swapped'},
+ 'xdebug-1.03': {'id': 'Xdebug-1.03'},
+ 'xerox': {'id': 'Xerox'},
+ 'xfig': {'id': 'Xfig'},
+ 'xfree86-1.1': {'id': 'XFree86-1.1'},
+ 'xinetd': {'id': 'xinetd'},
+ 'xkeyboard-config-zinoviev': {'id': 'xkeyboard-config-Zinoviev'},
+ 'xlock': {'id': 'xlock'},
+ 'xnet': {'id': 'Xnet'},
+ 'xpp': {'id': 'xpp'},
+ 'xskat': {'id': 'XSkat'},
+ 'xzoom': {'id': 'xzoom'},
+ 'ypl-1.0': {'id': 'YPL-1.0'},
+ 'ypl-1.1': {'id': 'YPL-1.1'},
+ 'zed': {'id': 'Zed'},
+ 'zeeff': {'id': 'Zeeff'},
+ 'zend-2.0': {'id': 'Zend-2.0'},
+ 'zimbra-1.3': {'id': 'Zimbra-1.3'},
+ 'zimbra-1.4': {'id': 'Zimbra-1.4'},
+ 'zlib': {'id': 'Zlib'},
+ 'zlib-acknowledgement': {'id': 'zlib-acknowledgement'},
+ 'zpl-1.1': {'id': 'ZPL-1.1'},
+ 'zpl-2.0': {'id': 'ZPL-2.0'},
+ 'zpl-2.1': {'id': 'ZPL-2.1'}}
diff --git a/flit_core/flit_core/common.py b/flit_core/flit_core/common.py
index 719e1072..22c188c8 100644
--- a/flit_core/flit_core/common.py
+++ b/flit_core/flit_core/common.py
@@ -336,6 +336,7 @@ class Metadata(object):
     maintainer = None
     maintainer_email = None
     license = None
+    license_expression = None
     description = None
     keywords = None
     download_url = None
@@ -399,7 +400,6 @@ def write_metadata_file(self, fp):
         optional_fields = [
             'Summary',
             'Home-page',
-            'License',
             'Keywords',
             'Author',
             'Author-email',
@@ -423,6 +423,17 @@ def write_metadata_file(self, fp):
                 value = '\n        '.join(value.splitlines())
                 fp.write(u"{}: {}\n".format(field, value))
 
+
+        license_expr = getattr(self, self._normalise_field_name("License-Expression"))
+        license = getattr(self, self._normalise_field_name("License"))
+        if license_expr:
+            # TODO: License-Expression requires Metadata-Version '2.4'
+            # Backfill it to the 'License' field for now
+            # fp.write(u'License-Expression: {}\n'.format(license_expr))
+            fp.write(u'License: {}\n'.format(license_expr))
+        elif license:
+            fp.write(u'License: {}\n'.format(license))
+
         for clsfr in self.classifiers:
             fp.write(u'Classifier: {}\n'.format(clsfr))
 
diff --git a/flit_core/flit_core/config.py b/flit_core/flit_core/config.py
index 2b2af868..3786001d 100644
--- a/flit_core/flit_core/config.py
+++ b/flit_core/flit_core/config.py
@@ -495,6 +495,14 @@ def _check_type(d, field_name, cls):
             "{} field should be {}, not {}".format(field_name, cls, type(d[field_name]))
         )
 
+def _check_types(d, field_name, cls_list) -> None:
+    if not isinstance(d[field_name], cls_list):
+        raise ConfigError(
+            "{} field should be {}, not {}".format(
+                field_name, ' or '.join(map(str, cls_list)), type(d[field_name])
+            )
+        )
+
 def _check_list_of_str(d, field_name):
     if not isinstance(d[field_name], list) or not all(
         isinstance(e, str) for e in d[field_name]
@@ -577,30 +585,38 @@ def read_pep621_metadata(proj, path) -> LoadedConfig:
 
     license_files = set()
     if 'license' in proj:
-        _check_type(proj, 'license', dict)
-        license_tbl = proj['license']
-        unrec_keys = set(license_tbl.keys()) - {'text', 'file'}
-        if unrec_keys:
-            raise ConfigError(
-                "Unrecognised keys in [project.license]: {}".format(unrec_keys)
-            )
+        _check_types(proj, 'license', (str, dict))
+        if isinstance(proj['license'], str):
+            md_dict['license_expression'] = normalize_license_expr(proj['license'])
+        else:
+            license_tbl = proj['license']
+            unrec_keys = set(license_tbl.keys()) - {'text', 'file'}
+            if unrec_keys:
+                raise ConfigError(
+                    "Unrecognised keys in [project.license]: {}".format(unrec_keys)
+                )
 
-        # TODO: Do something with license info.
-        # The 'License' field in packaging metadata is a brief description of
-        # a license, not the full text or a file path. PEP 639 will improve on
-        # how licenses are recorded.
-        if 'file' in license_tbl:
-            if 'text' in license_tbl:
+            # The 'License' field in packaging metadata is a brief description of
+            # a license, not the full text or a file path.
+            if 'file' in license_tbl:
+                if 'text' in license_tbl:
+                    raise ConfigError(
+                        "[project.license] should specify file or text, not both"
+                    )
+                license_f = license_tbl['file']
+                if isabs_ish(license_f):
+                    raise ConfigError(
+                        f"License file path ({license_f}) cannot be an absolute path"
+                    )
+                if not (path.parent / license_f).is_file():
+                    raise ConfigError(f"License file {license_f} does not exist")
+                license_files.add(license_tbl['file'])
+            elif 'text' in license_tbl:
+                pass
+            else:
                 raise ConfigError(
-                    "[project.license] should specify file or text, not both"
+                    "file or text field required in [project.license] table"
                 )
-            license_files.add(license_tbl['file'])
-        elif 'text' in license_tbl:
-            pass
-        else:
-            raise ConfigError(
-                "file or text field required in [project.license] table"
-            )
 
     if 'license-files' in proj:
         _check_type(proj, 'license-files', list)
@@ -635,6 +651,16 @@ def read_pep621_metadata(proj, path) -> LoadedConfig:
 
     if 'classifiers' in proj:
         _check_list_of_str(proj, 'classifiers')
+        classifiers = proj['classifiers']
+        license_expr = md_dict.get('license_expression', None)
+        if license_expr:
+            for cl in classifiers:
+                if not cl.startswith('License :: '):
+                    continue
+                raise ConfigError(
+                    "License classifier are deprecated in favor of the license expression. "
+                    "Remove the '{}' classifier".format(cl)
+                )
         md_dict['classifiers'] = proj['classifiers']
 
     if 'urls' in proj:
@@ -788,3 +814,36 @@ def isabs_ish(path):
     absolute paths, we also want to reject these odd halfway paths.
     """
     return os.path.isabs(path) or path.startswith(('/', '\\'))
+
+
+def normalize_license_expr(s: str):
+    """Validate & normalise an SPDX license expression
+
+    For now this only handles simple expressions (referring to 1 license)
+    """
+    from ._spdx_data import licenses
+    ls = s.lower()
+    if ls.startswith('licenseref-'):
+        ref = s.partition('-')[2]
+        if re.match(r'([a-zA-Z0-9\-.])+$', ref):
+            # Normalise case of LicenseRef, leave the rest alone
+            return "LicenseRef-" + ref
+        raise ConfigError(
+            "LicenseRef- license expression can only contain ASCII letters "
+            "& digits, - and ."
+        )
+
+    or_later = s.endswith('+')
+    if or_later:
+        ls = ls[:-1]
+
+    try:
+        info = licenses[ls]
+    except KeyError:
+        if os.environ.get('FLIT_ALLOW_INVALID'):
+            log.warning("Invalid license ID {!r} allowed by FLIT_ALLOW_INVALID"
+                        .format(s))
+            return s
+        raise ConfigError(f"{s!r} is not a recognised SPDX license ID")
+
+    return info['id'] + ('+' if or_later else '')
diff --git a/flit_core/pyproject.toml b/flit_core/pyproject.toml
index 53c47343..11702af8 100644
--- a/flit_core/pyproject.toml
+++ b/flit_core/pyproject.toml
@@ -12,9 +12,9 @@ description = "Distribution-building parts of Flit. See flit package for more in
 dependencies = []
 requires-python = '>=3.6'
 readme = "README.rst"
+license = "BSD-3-Clause"
 license-files = ["LICENSE*", "flit_core/vendor/**/LICENSE*"]
 classifiers = [
-    "License :: OSI Approved :: BSD License",
     "Topic :: Software Development :: Libraries :: Python Modules",
 ]
 dynamic = ["version"]
diff --git a/flit_core/tests_core/test_common.py b/flit_core/tests_core/test_common.py
index 824fa5de..3823e27d 100644
--- a/flit_core/tests_core/test_common.py
+++ b/flit_core/tests_core/test_common.py
@@ -205,3 +205,27 @@ def test_metadata_2_3_provides_extra(provides_extra, expected_result):
     msg = email.parser.Parser(policy=email.policy.compat32).parse(sio)
     assert msg['Provides-Extra'] == expected_result
     assert not msg.defects
+
+@pytest.mark.parametrize(
+    ('value', 'expected_license', 'expected_license_expression'),
+    [
+        ({'license': 'MIT'}, 'MIT', None),
+        ({'license_expression': 'MIT'}, 'MIT', None),  # TODO Metadata 2.4
+        ({'license_expression': 'Apache-2.0'}, 'Apache-2.0', None)  # TODO Metadata 2.4
+    ],
+)
+def test_metadata_license(value, expected_license, expected_license_expression):
+    d = {
+        'name': 'foo',
+        'version': '1.0',
+        **value,
+    }
+    md = Metadata(d)
+    sio = StringIO()
+    md.write_metadata_file(sio)
+    sio.seek(0)
+
+    msg = email.parser.Parser(policy=email.policy.compat32).parse(sio)
+    assert msg.get('License') == expected_license
+    assert msg.get('License-Expression') == expected_license_expression
+    assert not msg.defects
diff --git a/flit_core/tests_core/test_config.py b/flit_core/tests_core/test_config.py
index 2558a3b1..b25bcc04 100644
--- a/flit_core/tests_core/test_config.py
+++ b/flit_core/tests_core/test_config.py
@@ -140,6 +140,12 @@ def test_bad_include_paths(path, err_match):
     ({'license': {'fromage': 2}}, '[Uu]nrecognised'),
     ({'license': {'file': 'LICENSE', 'text': 'xyz'}}, 'both'),
     ({'license': {}}, 'required'),
+    ({'license': 1}, "license field should be <class 'str'> or <class 'dict'>, not <class 'int'>"),
+    # ({'license': "MIT License"}, "Invalid license expression: 'MIT License'"),  # TODO
+    (
+        {'license': 'MIT', 'classifiers': ['License :: OSI Approved :: MIT License']},
+        "License classifier are deprecated in favor of the license expression",
+    ),
     ({'license-files': 1}, r"\blist\b"),
     ({'license-files': ["/LICENSE"]}, r"'/LICENSE'.+must not start with '/'"),
     ({'license-files': ["../LICENSE"]}, r"'../LICENSE'.+must not contain '..'"),
@@ -202,6 +208,37 @@ def test_bad_pep621_readme(readme, err_match):
         config.read_pep621_metadata(proj, samples_dir / 'pep621' / 'pyproject.toml')
 
 
+@pytest.mark.parametrize(('value', 'license_expression'), [
+    # Accept and normalize valid SPDX expressions for 'license = ...'
+    ("mit",  "MIT"),
+    ("apache-2.0", "Apache-2.0"),
+    ("APACHE-2.0+", "Apache-2.0+"),
+    # TODO: compound expressions
+    #("mit and (apache-2.0 or bsd-2-clause)", "MIT AND (Apache-2.0 OR BSD-2-Clause)"),
+    # LicenseRef expressions: only the LicenseRef is normalised
+    ("LiceNseref-Public-DoMain", "LicenseRef-Public-DoMain"),
+])
+def test_license_expr(value, license_expression):
+    proj = {
+        'name': 'module1', 'version': '1.0', 'description': 'x', 'license': value
+    }
+    info = config.read_pep621_metadata(proj, samples_dir / 'pep621' / 'pyproject.toml')
+    assert 'license' not in info.metadata
+    assert info.metadata['license_expression'] == license_expression
+
+def test_license_expr_error():
+    proj = {
+        'name': 'module1', 'version': '1.0', 'description': 'x',
+        'license': 'LicenseRef-foo_bar',  # Underscore not allowed
+    }
+    with pytest.raises(config.ConfigError, match="can only contain"):
+        config.read_pep621_metadata(proj, samples_dir / 'pep621' / 'pyproject.toml')
+
+    proj['license'] = "BSD-33-Clause"  # Not a real license
+    with pytest.raises(config.ConfigError, match="recognised"):
+        config.read_pep621_metadata(proj, samples_dir / 'pep621' / 'pyproject.toml')
+
+
 def test_license_file_defaults_with_old_metadata():
     metadata = {'module': 'mymod', 'author': ''}
     info = config._prep_metadata(metadata, samples_dir / 'pep621_license_files' / 'pyproject.toml')
diff --git a/prepare_license_list.py b/prepare_license_list.py
new file mode 100644
index 00000000..76e62d4c
--- /dev/null
+++ b/prepare_license_list.py
@@ -0,0 +1,24 @@
+# Call with path to SPDX license-list-data repo, cloned from:
+#   https://github.com/spdx/license-list-data
+
+import json
+import pprint
+import sys
+from pathlib import Path
+
+list_data_repo = Path(sys.argv[1])
+with (list_data_repo / 'json' / 'licenses.json').open('rb') as f:
+    licenses_json = json.load(f)
+
+condensed = {
+    l['licenseId'].lower() : {'id': l['licenseId']}
+    for l in licenses_json['licenses']
+    if not l['isDeprecatedLicenseId']
+}
+
+with Path('flit_core', 'flit_core', '_spdx_data.py').open('w') as f:
+    f.write("# This file is generated from SPDX license data; don't edit it manually.\n\n")
+
+    f.write("licenses = \\\n")
+    pprint.pprint(condensed, f)
+