diff --git a/docs/changelog/1129.deprecation.rst b/docs/changelog/1129.deprecation.rst
new file mode 100644
index 000000000..f9915c19c
--- /dev/null
+++ b/docs/changelog/1129.deprecation.rst
@@ -0,0 +1 @@
+Whitelisting of externals will be mandatory in tox 4: issue a deprecation warning as part of the already existing warning - by :user:`obestwalter`
diff --git a/src/tox/venv.py b/src/tox/venv.py
index d3e9954c7..68a54439c 100755
--- a/src/tox/venv.py
+++ b/src/tox/venv.py
@@ -174,7 +174,8 @@ def _check_external_allowed_and_warn(self, path):
                 "  cmd: {}\n"
                 "  env: {}\n"
                 "Maybe you forgot to specify a dependency? "
-                "See also the whitelist_externals envconfig setting.".format(
+                "See also the whitelist_externals envconfig setting.\n\n"
+                "DEPRECATION WARNING: this will be an error in tox 4 and above!".format(
                     path, self.envconfig.envdir
                 )
             )