diff --git a/docs/releases/6.3.md b/docs/releases/6.3.md
index f88008739f6e..c99956d3a33d 100644
--- a/docs/releases/6.3.md
+++ b/docs/releases/6.3.md
@@ -91,3 +91,9 @@ To disable the automatic preview update feature, set [`WAGTAIL_AUTO_UPDATE_PREVI
 ## Upgrade considerations - changes affecting Wagtail customisations
 
 ## Upgrade considerations - changes to undocumented internals
+
+### Deprecation of the `{% locales %}` template tag
+
+The undocumented `locales` template tag will be removed in a future release.
+
+If access to JSON locales within JavaScript is needed, use `window.wagtailConfig.LOCALES` instead.
diff --git a/wagtail/admin/templatetags/wagtailadmin_tags.py b/wagtail/admin/templatetags/wagtailadmin_tags.py
index a54c10494f24..c63fbdb0712f 100644
--- a/wagtail/admin/templatetags/wagtailadmin_tags.py
+++ b/wagtail/admin/templatetags/wagtailadmin_tags.py
@@ -867,7 +867,15 @@ def locales(serialize=True):
         }
         for locale in Locale.objects.all()
     ]
-    return json.dumps(result) if serialize else result
+
+    if serialize:
+        warn(
+            "The `locales` template tag will be removed in a future release.",
+            category=RemovedInWagtail70Warning,
+        )
+        return json.dumps(result)
+
+    return result
 
 
 @register.simple_tag