diff --git a/airlock/business_logic.py b/airlock/business_logic.py
index 21c59ae5..9d68e336 100644
--- a/airlock/business_logic.py
+++ b/airlock/business_logic.py
@@ -266,6 +266,14 @@ def all_files_set(self):
             for request_file in filegroup.files.values()
         }
 
+    def output_files_set(self):
+        """Return the relpaths for output files on the request"""
+        return {
+            request_file.relpath
+            for filegroup in self.filegroups.values()
+            for request_file in filegroup.output_files
+        }
+
     def is_supporting_file(self, urlpath: UrlPath):
         try:
             return self.get_request_file(urlpath).filetype == RequestFileType.SUPPORTING
@@ -633,8 +641,10 @@ def _verify_permission_to_review_file(
                 "only an output checker can approve a file"
             )
 
-        if path not in release_request.file_set():
-            raise self.ApprovalPermissionDenied("file is not part of the request")
+        if path not in release_request.output_files_set():
+            raise self.ApprovalPermissionDenied(
+                "file is not an output file on this request"
+            )
 
     def approve_file(self, release_request: ReleaseRequest, user: User, path: Path):
         """ "Approve a file"""
diff --git a/tests/unit/test_business_logic.py b/tests/unit/test_business_logic.py
index f91a22d4..caca33f3 100644
--- a/tests/unit/test_business_logic.py
+++ b/tests/unit/test_business_logic.py
@@ -610,6 +610,20 @@ def test_approve_file_not_part_of_request(bll):
         bll.approve_file(release_request, checker, bad_path)
 
 
+def test_approve_supporting_file(bll):
+    release_request, path, author = setup_empty_release_request()
+    checker = factories.create_user("checker", [], True)
+
+    bll.add_file_to_request(
+        release_request, path, author, filetype=RequestFileType.SUPPORTING
+    )
+    bll.set_status(
+        release_request=release_request, to_status=RequestStatus.SUBMITTED, user=author
+    )
+    with pytest.raises(bll.ApprovalPermissionDenied):
+        bll.approve_file(release_request, checker, path)
+
+
 def test_approve_file(bll):
     release_request, path, author = setup_empty_release_request()
     checker = factories.create_user("checker", [], True)