diff --git a/tests/rest/client/v1/test_login.py b/tests/rest/client/v1/test_login.py
index aed8853d6e9d..1856c7ffd5a6 100644
--- a/tests/rest/client/v1/test_login.py
+++ b/tests/rest/client/v1/test_login.py
@@ -257,7 +257,7 @@ def _delete_device(self, access_token, user_id, password, device_id):
         self.assertEquals(channel.code, 200, channel.result)
 
 
-class CASRedirectConfirmTestCase(unittest.HomeserverTestCase):
+class CASTestCase(unittest.HomeserverTestCase):
 
     servlets = [
         login.register_servlets,
@@ -274,6 +274,9 @@ def make_homeserver(self, reactor, clock):
             "service_url": "https://matrix.goodserver.com:8448",
         }
 
+        cas_user_id = "username"
+        self.user_id = "@%s:test" % cas_user_id
+
         async def get_raw(uri, args):
             """Return an example response payload from a call to the `/proxyValidate`
             endpoint of a CAS server, copied from
@@ -282,10 +285,11 @@ async def get_raw(uri, args):
             This needs to be returned by an async function (as opposed to set as the
             mock's return value) because the corresponding Synapse code awaits on it.
             """
-            return """
+            return (
+                """
                 <cas:serviceResponse xmlns:cas='http://www.yale.edu/tp/cas'>
                   <cas:authenticationSuccess>
-                      <cas:user>username</cas:user>
+                      <cas:user>%s</cas:user>
                       <cas:proxyGrantingTicket>PGTIOU-84678-8a9d...</cas:proxyGrantingTicket>
                       <cas:proxies>
                           <cas:proxy>https://proxy2/pgtUrl</cas:proxy>
@@ -294,6 +298,8 @@ async def get_raw(uri, args):
                   </cas:authenticationSuccess>
                 </cas:serviceResponse>
             """
+                % cas_user_id
+            )
 
         mocked_http_client = Mock(spec=["get_raw"])
         mocked_http_client.get_raw.side_effect = get_raw
@@ -304,6 +310,9 @@ async def get_raw(uri, args):
 
         return self.hs
 
+    def prepare(self, reactor, clock, hs):
+        self.deactivate_account_handler = hs.get_deactivate_account_handler()
+
     def test_cas_redirect_confirm(self):
         """Tests that the SSO login flow serves a confirmation page before redirecting a
         user to the redirect URL.
@@ -370,3 +379,30 @@ def _test_redirect(self, redirect_url):
         self.assertEqual(channel.code, 302)
         location_headers = channel.headers.getRawHeaders("Location")
         self.assertEqual(location_headers[0][: len(redirect_url)], redirect_url)
+
+    @override_config({"sso": {"client_whitelist": ["https://legit-site.com/"]}})
+    def test_deactivated_user(self):
+        """Logging in as a deactivated account should error."""
+        redirect_url = "https://legit-site.com/"
+
+        # First login (to create the user).
+        self._test_redirect(redirect_url)
+
+        # Deactivate the account.
+        self.get_success(
+            self.deactivate_account_handler.deactivate_account(self.user_id, False)
+        )
+
+        # Request the CAS ticket.
+        cas_ticket_url = (
+            "/_matrix/client/r0/login/cas/ticket?redirectUrl=%s&ticket=ticket"
+            % (urllib.parse.quote(redirect_url))
+        )
+
+        # Get Synapse to call the fake CAS and serve the template.
+        request, channel = self.make_request("GET", cas_ticket_url)
+        self.render(request)
+
+        # Because the user is deactivated they are served an error template.
+        self.assertEqual(channel.code, 403)
+        self.assertIn(b"SSO account deactivated", channel.result["body"])