diff --git a/changelog.d/20180.bugfix b/changelog.d/20180.bugfix
new file mode 100644
index 0000000000..72f4fc51d0
--- /dev/null
+++ b/changelog.d/20180.bugfix
@@ -0,0 +1 @@
+Return the stable `M_APPSERVICE_LOGIN_UNSUPPORTED` error code, added in Matrix 1.17, instead of its unstable MSC4190-prefixed identifier.
diff --git a/synapse/api/errors.py b/synapse/api/errors.py
index 78fd8c3a60..00b5fa9a76 100644
--- a/synapse/api/errors.py
+++ b/synapse/api/errors.py
@@ -143,7 +143,7 @@ class Codes(str, Enum):
     INVITE_BLOCKED = "M_INVITE_BLOCKED"
 
     # Part of MSC4190
-    APPSERVICE_LOGIN_UNSUPPORTED = "IO.ELEMENT.MSC4190.M_APPSERVICE_LOGIN_UNSUPPORTED"
+    APPSERVICE_LOGIN_UNSUPPORTED = "M_APPSERVICE_LOGIN_UNSUPPORTED"
 
     # Part of MSC4306: Thread Subscriptions
     MSC4306_CONFLICTING_UNSUBSCRIPTION = (
diff --git a/tests/handlers/test_oauth_delegation.py b/tests/handlers/test_oauth_delegation.py
index 995a1134b2..71f83607ff 100644
--- a/tests/handlers/test_oauth_delegation.py
+++ b/tests/handlers/test_oauth_delegation.py
@@ -587,6 +587,34 @@ class DisabledEndpointsTestCase(HomeserverTestCase):
             "POST", "/_matrix/client/v3/register/msisdn/requestToken"
         )
 
+    def test_appservice_registration_without_inhibit_login(self) -> None:
+        """Test that appservice registration without `inhibit_login` is rejected
+        with the `M_APPSERVICE_LOGIN_UNSUPPORTED` errcode."""
+        appservice = ApplicationService(
+            token="i_am_an_app_service",
+            id="1234",
+            namespaces={"users": [{"regex": r"@alice:.+", "exclusive": True}]},
+            sender=UserID.from_string("@as_main:test"),
+        )
+
+        self.hs.get_datastores().main.services_cache = [appservice]
+        channel = self.make_request(
+            "POST",
+            "/_matrix/client/v3/register",
+            {
+                "username": "alice",
+                "type": "m.login.application_service",
+            },
+            shorthand=False,
+            access_token="i_am_an_app_service",
+        )
+        self.assertEqual(channel.code, 400, channel.json_body)
+        self.assertEqual(
+            channel.json_body.get("errcode"),
+            "M_APPSERVICE_LOGIN_UNSUPPORTED",
+            channel.json_body,
+        )
+
     def test_session_management_endpoints_removed(self) -> None:
         """Test that session management endpoints that were removed in MSC2964 are no longer available."""
         self.expect_unrecognized("GET", "/_matrix/client/v3/login")
diff --git a/tests/rest/client/test_login.py b/tests/rest/client/test_login.py
index d83604a696..f1639bfeee 100644
--- a/tests/rest/client/test_login.py
+++ b/tests/rest/client/test_login.py
@@ -1521,7 +1521,7 @@ class AppserviceLoginRestServletTestCase(unittest.HomeserverTestCase):
         self.assertEqual(channel.code, 200, msg=channel.result)
 
     def test_login_appservice_msc4190_fail(self) -> None:
-        """Test that an appservice user can use /login"""
+        """Test that an appservice with MSC4190 enabled can't use /login"""
         self.register_appservice_user(
             "as3_user_alice", self.msc4190_service.token, inhibit_login=True
         )
@@ -1537,7 +1537,7 @@ class AppserviceLoginRestServletTestCase(unittest.HomeserverTestCase):
         self.assertEqual(channel.code, 400, msg=channel.result)
         self.assertEqual(
             channel.json_body.get("errcode"),
-            Codes.APPSERVICE_LOGIN_UNSUPPORTED,
+            "M_APPSERVICE_LOGIN_UNSUPPORTED",
             channel.json_body,
         )
 
diff --git a/tests/rest/client/test_register.py b/tests/rest/client/test_register.py
index 1b04ea9d5c..67f78ee24f 100644
--- a/tests/rest/client/test_register.py
+++ b/tests/rest/client/test_register.py
@@ -183,7 +183,7 @@ class RegisterRestServletTestCase(unittest.HomeserverTestCase):
         self.assertEqual(channel.code, 400, channel.json_body)
         self.assertEqual(
             channel.json_body.get("errcode"),
-            Codes.APPSERVICE_LOGIN_UNSUPPORTED,
+            "M_APPSERVICE_LOGIN_UNSUPPORTED",
             channel.json_body,
         )