diff --git a/flit/buildapi.py b/flit/buildapi.py
index 0de2e9d4..c7f640f4 100644
--- a/flit/buildapi.py
+++ b/flit/buildapi.py
@@ -1 +1,6 @@
+from warnings import warn
+warn('A package has specified `build-backend = "flit.buildapi"` and is being '
+     'built with Flit >= 3.10. This is likely to break in a future version. '
+     'Please change the backend to flit_core.buildapi, and/or specify a '
+     'maximum version of Flit.')
 from flit_core.buildapi import *