diff --git a/changelog.d/10520.misc b/changelog.d/10520.misc
new file mode 100644
index 000000000000..a911e165da80
--- /dev/null
+++ b/changelog.d/10520.misc
@@ -0,0 +1 @@
+Send and handle cross-signing messages using the stable prefix.
diff --git a/synapse/handlers/e2e_keys.py b/synapse/handlers/e2e_keys.py
index 60c11e3d2128..b2554bda045a 100644
--- a/synapse/handlers/e2e_keys.py
+++ b/synapse/handlers/e2e_keys.py
@@ -65,8 +65,12 @@ def __init__(self, hs: "HomeServer"):
         else:
             # Only register this edu handler on master as it requires writing
             # device updates to the db
-            #
-            # FIXME: switch to m.signing_key_update when MSC1756 is merged into the spec
+            federation_registry.register_edu_handler(
+                "m.signing_key_update",
+                self._edu_updater.incoming_signing_key_update,
+            )
+            # also handle the unstable version
+            # FIXME: remove this when enough servers have upgraded
             federation_registry.register_edu_handler(
                 "org.matrix.signing_key_update",
                 self._edu_updater.incoming_signing_key_update,
diff --git a/synapse/storage/databases/main/devices.py b/synapse/storage/databases/main/devices.py
index d5a4a661cd1a..838a2a6a3dd0 100644
--- a/synapse/storage/databases/main/devices.py
+++ b/synapse/storage/databases/main/devices.py
@@ -274,7 +274,9 @@ async def get_device_updates_by_remote(
         # add the updated cross-signing keys to the results list
         for user_id, result in cross_signing_keys_by_user.items():
             result["user_id"] = user_id
-            # FIXME: switch to m.signing_key_update when MSC1756 is merged into the spec
+            results.append(("m.signing_key_update", result))
+            # also send the unstable version
+            # FIXME: remove this when enough servers have upgraded
             results.append(("org.matrix.signing_key_update", result))
 
         return now_stream_id, results
diff --git a/tests/federation/test_federation_sender.py b/tests/federation/test_federation_sender.py
index b457dad6d263..b2376e2db925 100644
--- a/tests/federation/test_federation_sender.py
+++ b/tests/federation/test_federation_sender.py
@@ -266,7 +266,8 @@ def test_upload_signatures(self):
         )
 
         # expect signing key update edu
-        self.assertEqual(len(self.edus), 1)
+        self.assertEqual(len(self.edus), 2)
+        self.assertEqual(self.edus.pop(0)["edu_type"], "m.signing_key_update")
         self.assertEqual(self.edus.pop(0)["edu_type"], "org.matrix.signing_key_update")
 
         # sign the devices
@@ -491,7 +492,7 @@ def check_signing_key_update_txn(
     ) -> None:
         """Check that the txn has an EDU with a signing key update."""
         edus = txn["edus"]
-        self.assertEqual(len(edus), 1)
+        self.assertEqual(len(edus), 2)
 
     def generate_and_upload_device_signing_key(
         self, user_id: str, device_id: str