diff --git a/cylc/flow/config.py b/cylc/flow/config.py
index f5ce67488e6..30577f0ea66 100644
--- a/cylc/flow/config.py
+++ b/cylc/flow/config.py
@@ -81,6 +81,7 @@
     is_relative_to,
 )
 from cylc.flow.print_tree import print_tree
+from cylc.flow.task_qualifiers import ALT_QUALIFIERS
 from cylc.flow.simulation import configure_sim_modes
 from cylc.flow.subprocctx import SubFuncContext
 from cylc.flow.task_events_mgr import (
@@ -1136,6 +1137,15 @@ def _check_completion_expression(self, task_name: str, expr: str) -> None:
                     ' expressions, use "succeeded or failed".'
                 )
 
+            for alt_qualifier, qualifier in ALT_QUALIFIERS.items():
+                if alt_qualifier in error:
+                    raise WorkflowConfigError(
+                        f'Error in [runtime][{task_name}]completion:'
+                        f'\n  {expr}'
+                        f'\nUse "{qualifier}" not "{alt_qualifier}" '
+                        'in completion expressions.'
+                    )
+
             raise WorkflowConfigError(
                 # NOTE: str(exc) == "name 'x' is not defined" tested in
                 # tests/integration/test_optional_outputs.py
diff --git a/tests/integration/validate/test_outputs.py b/tests/integration/validate/test_outputs.py
index 3ddeba0e96d..63e5df5a248 100644
--- a/tests/integration/validate/test_outputs.py
+++ b/tests/integration/validate/test_outputs.py
@@ -219,6 +219,12 @@ def test_messages(messages, valid, flow, validate):
             'but required in the completion expression',
             id='failed-implicitly-optional-in-graph-required-in-completion',
         ),
+        pytest.param(
+            'foo',
+            '(succeed and x) or failed',
+            'Use "succeeded" not "succeed" in completion expressions',
+            id='alt-qualifier',
+        ),
     ]
 )
 def test_completion_expression_invalid(