diff --git a/changelog.d/7625.misc b/changelog.d/7625.misc
new file mode 100644
index 000000000000..4c61d8d99f15
--- /dev/null
+++ b/changelog.d/7625.misc
@@ -0,0 +1 @@
+Check if the localpart of a Matrix ID is reserved for guest users earlier in the registration flow, as well as when responding to requests to `/register/available`.
diff --git a/synapse/handlers/register.py b/synapse/handlers/register.py
index a6178e74a19b..55a03e53ead4 100644
--- a/synapse/handlers/register.py
+++ b/synapse/handlers/register.py
@@ -128,6 +128,15 @@ def check_username(self, localpart, guest_access_token=None, assigned_user_id=No
                     errcode=Codes.FORBIDDEN,
                 )
 
+        if guest_access_token is None:
+            try:
+                int(localpart)
+                raise SynapseError(
+                    400, "Numeric user IDs are reserved for guest users."
+                )
+            except ValueError:
+                pass
+
     @defer.inlineCallbacks
     def register_user(
         self,
@@ -170,15 +179,6 @@ def register_user(
 
             was_guest = guest_access_token is not None
 
-            if not was_guest:
-                try:
-                    int(localpart)
-                    raise SynapseError(
-                        400, "Numeric user IDs are reserved for guest users."
-                    )
-                except ValueError:
-                    pass
-
             user = UserID(localpart, self.hs.hostname)
             user_id = user.to_string()