diff --git a/lib/iris/fileformats/_nc_load_rules/actions.py b/lib/iris/fileformats/_nc_load_rules/actions.py
index 8351a7ea3b..e3b63d41d2 100644
--- a/lib/iris/fileformats/_nc_load_rules/actions.py
+++ b/lib/iris/fileformats/_nc_load_rules/actions.py
@@ -43,11 +43,15 @@
 from functools import wraps
 import warnings
 
+from iris.config import get_logger
 import iris.fileformats.cf
 import iris.fileformats.pp as pp
 
 from . import helpers as hh
 
+# Configure the logger.
+logger = get_logger(__name__, fmt="[%(funcName)s]")
+
 
 def _default_rulenamesfunc(func_name):
     # A simple default function to deduce the rules-name from an action-name.
@@ -397,9 +401,9 @@ def action_ukmo_stash(engine):
             engine.cube.attributes[attr_name] = attr_value
             msg = (
                 "Unable to set attribute STASH as not a valid MSI "
-                f'string "mXXsXXiXXX", got {attr_value}'
+                f'string "mXXsXXiXXX", got "{attr_value}"'
             )
-            warnings.warn(msg)
+            logger.debug(msg)
         else:
             engine.cube.attributes["STASH"] = stash_code
 
diff --git a/lib/iris/tests/test_netcdf.py b/lib/iris/tests/test_netcdf.py
index 9348a66935..61dfc73580 100644
--- a/lib/iris/tests/test_netcdf.py
+++ b/lib/iris/tests/test_netcdf.py
@@ -632,8 +632,7 @@ def test_bad_um_stash_source(self):
         command = "ncgen -o {} {}".format(nc_path, cdl_path)
         check_call(command, shell=True)
         # Load with iris.fileformats.netcdf.load_cubes, and check expected content.
-        with self.assertWarns(UserWarning):
-            cubes = list(nc_load_cubes(nc_path))
+        cubes = list(nc_load_cubes(nc_path))
         self.assertEqual(len(cubes), 1)
         self.assertFalse(hasattr(cubes[0].attributes, "STASH"))
         self.assertEqual(
diff --git a/lib/iris/tests/unit/fileformats/nc_load_rules/actions/test__miscellaneous.py b/lib/iris/tests/unit/fileformats/nc_load_rules/actions/test__miscellaneous.py
index 4ed90fd79a..a8e44747dd 100644
--- a/lib/iris/tests/unit/fileformats/nc_load_rules/actions/test__miscellaneous.py
+++ b/lib/iris/tests/unit/fileformats/nc_load_rules/actions/test__miscellaneous.py
@@ -83,14 +83,16 @@ def test_stash_altname(self):
         self.check_result(cube, stashcode=self.stashcode)
 
     def test_stash_empty(self):
-        msg = "Expected STASH code MSI string"
-        with self.assertRaisesRegex(ValueError, msg):
-            self.run_testcase(ukmo__um_stash_source="")
+        value = ""
+        cube = self.run_testcase(ukmo__um_stash_source=value)
+        self.assertNotIn("STASH", cube.attributes)
+        self.assertEqual(cube.attributes["ukmo__um_stash_source"], value)
 
     def test_stash_invalid(self):
-        msg = "Expected STASH code MSI string"
-        with self.assertRaisesRegex(ValueError, msg):
-            self.run_testcase(ukmo__um_stash_source="XXX")
+        value = "XXX"
+        cube = self.run_testcase(ukmo__um_stash_source="XXX")
+        self.assertNotIn("STASH", cube.attributes)
+        self.assertEqual(cube.attributes["ukmo__um_stash_source"], value)
 
     def test_processflags_single(self):
         cube = self.run_testcase(ukmo__process_flags="this")