diff --git a/docs/reference/configuration/keywords.rst b/docs/reference/configuration/keywords.rst
index 43f914d0e30..e29798fe0cf 100644
--- a/docs/reference/configuration/keywords.rst
+++ b/docs/reference/configuration/keywords.rst
@@ -477,8 +477,8 @@ Commonly used keywords
         will be replaced by the realization number and iteration number when ERT creates the folders.
         By default, RUNPATH is set to "simulations/realization-<IENS>/iter-<ITER>".
 
-        Deprecated syntax still allow use of two %d specifers. Use of less than two %d is prohibited.
-        The behaviour is identical to the default substitution.
+        Deprecated syntax still allow use of two `%d` specifers. Use of more than two `%d` specifiers,
+        using multiple `<IENS>` or `<ITER>` keywords or mixing styles is prohibited.
 
         *Example:*
 
diff --git a/src/ert/config/model_config.py b/src/ert/config/model_config.py
index bdde2ab1089..2f2357fba7d 100644
--- a/src/ert/config/model_config.py
+++ b/src/ert/config/model_config.py
@@ -54,11 +54,31 @@ def __init__(
         self.history_source = history_source
         self.jobname_format_string = _replace_runpath_format(jobname_format_string)
         self.eclbase_format_string = _replace_runpath_format(eclbase_format_string)
-        self.runpath_format_string = _replace_runpath_format(runpath_format_string)
 
-        if self.runpath_format_string is not None and not any(
-            x in self.runpath_format_string for x in ["<ITER>", "<IENS>"]
+        # do not combine styles
+        if "%d" in runpath_format_string and any(
+            x in runpath_format_string for x in ["<ITER>", "<IENS>"]
         ):
+            raise ConfigValidationError(
+                f"RUNPATH cannot combine deprecated and new style placeholders: `{runpath_format_string}`. Valid example `{DEFAULT_RUNPATH}`"
+            )
+
+        # do not allow multiple occurrences
+        for kw in ["<ITER>", "<IENS>"]:
+            if runpath_format_string.count(kw) > 1:
+                raise ConfigValidationError(
+                    f"RUNPATH cannot contain multiple {kw} placeholders: `{runpath_format_string}`. Valid example `{DEFAULT_RUNPATH}`"
+                )
+
+        # do not allow too many placeholders
+        if runpath_format_string.count("%d") > 2:
+            raise ConfigValidationError(
+                f"RUNPATH cannot contain more than two value placeholders: `{runpath_format_string}`. Valid example `{DEFAULT_RUNPATH}`"
+            )
+
+        self.runpath_format_string = _replace_runpath_format(runpath_format_string)
+
+        if not any(x in self.runpath_format_string for x in ["<ITER>", "<IENS>"]):
             logger.warning(
                 "RUNPATH keyword contains no value placeholders: "
                 f"`{runpath_format_string}`. Valid example: "
diff --git a/tests/unit_tests/test_run_path_creation.py b/tests/unit_tests/test_run_path_creation.py
index 4b716e1c4f5..3422934ac5f 100644
--- a/tests/unit_tests/test_run_path_creation.py
+++ b/tests/unit_tests/test_run_path_creation.py
@@ -5,7 +5,7 @@
 
 import pytest
 
-from ert.config import ErtConfig
+from ert.config import ConfigValidationError, ErtConfig
 from ert.enkf_main import create_run_path, ensemble_context, sample_prior
 from ert.run_context import RunContext
 from ert.runpaths import Runpaths
@@ -469,3 +469,53 @@ def test_num_cpu_subst(monkeypatch, tmp_path, append, numcpu, storage):
 
     with open("simulations/realization-0/iter-0/jobs.json", encoding="utf-8") as f:
         assert f'"argList": ["{numcpu}"]' in f.read()
+
+
+@pytest.mark.parametrize(
+    "run_path, expected_raise, msg",
+    [
+        ("simulations/realization-<IENS>/iter-<ITER>", False, ""),
+        ("simulations/realization-%d/iter-%d", False, ""),
+        ("simulations/realization-%d", False, ""),
+        (
+            "simulations/realization-<IENS>/iter-%d",
+            True,
+            "RUNPATH cannot combine deprecated ",
+        ),
+        (
+            "simulations/realization-<IENS>/iter-<IENS>",
+            True,
+            "RUNPATH cannot contain multiple <IENS>",
+        ),
+        (
+            "simulations/realization-<ITER>/iter-<ITER>",
+            True,
+            "RUNPATH cannot contain multiple <ITER>",
+        ),
+        (
+            "simulations/realization-%d/iter-%d/more-%d",
+            True,
+            "RUNPATH cannot contain more than two",
+        ),
+    ],
+)
+@pytest.mark.usefixtures("use_tmpdir")
+def test_that_runpaths_are_raised_when_invalid(run_path, expected_raise, msg):
+    """
+    This checks that RUNPATH does not include too many or few substitution placeholders
+    """
+    config_text = dedent(
+        """
+        NUM_REALIZATIONS 2
+        """
+    )
+    Path("config.ert").write_text(config_text + f"RUNPATH {run_path}", encoding="utf-8")
+
+    if expected_raise:
+        with pytest.raises(
+            ConfigValidationError,
+            match=f"{msg}.*{run_path}`.",
+        ):
+            _ = ErtConfig.from_file("config.ert")
+    else:
+        _ = ErtConfig.from_file("config.ert")