diff --git a/bot_bottle/git_gate.py b/bot_bottle/git_gate.py
index 0c3b09f..1338b80 100644
--- a/bot_bottle/git_gate.py
+++ b/bot_bottle/git_gate.py
@@ -300,6 +300,8 @@ while IFS=' ' read -r old new ref; do
   [ -z "$ref" ] && continue
   if [ "$new" = "$zero" ]; then
     refspec=":$ref"
+  elif [ "$old" != "$zero" ] && ! git merge-base --is-ancestor "$old" "$new" 2>/dev/null; then
+    refspec="+$new:$ref"
   else
     refspec="$new:$ref"
   fi
diff --git a/tests/unit/test_git_gate.py b/tests/unit/test_git_gate.py
index 11e96ea..2e1de27 100644
--- a/tests/unit/test_git_gate.py
+++ b/tests/unit/test_git_gate.py
@@ -181,6 +181,13 @@ class TestHookRender(unittest.TestCase):
         self.assertIn("BatchMode=yes", hook)
         self.assertIn("ConnectTimeout=", hook)
 
+    def test_force_push_uses_plus_refspec(self):
+        # A non-fast-forward push (old != zero, new not a descendant of old)
+        # must forward +$new:$ref so the upstream accepts the force push.
+        hook = git_gate_render_hook()
+        self.assertIn('git merge-base --is-ancestor "$old" "$new"', hook)
+        self.assertIn('refspec="+$new:$ref"', hook)
+
     def test_forward_preserves_push_options(self):
         # Git exposes push options to pre-receive hooks as
         # GIT_PUSH_OPTION_COUNT + indexed GIT_PUSH_OPTION_N variables.