diff --git a/hydra/errors.py b/hydra/errors.py
index 8134188770..16af432a46 100644
--- a/hydra/errors.py
+++ b/hydra/errors.py
@@ -37,3 +37,5 @@ def __init__(
 
 
 class HydraDeprecationError(HydraException): ...
+
+class UnsupportedExtensionException(HydraException): ...
diff --git a/hydra/plugins/config_source.py b/hydra/plugins/config_source.py
index 182a3ec48a..f26149f305 100644
--- a/hydra/plugins/config_source.py
+++ b/hydra/plugins/config_source.py
@@ -10,7 +10,7 @@
 from hydra._internal.deprecation_warning import deprecation_warning
 from hydra.core.default_element import InputDefault
 from hydra.core.object_type import ObjectType
-from hydra.errors import HydraException
+from hydra.errors import HydraException, UnsupportedExtensionException
 from hydra.plugins.plugin import Plugin
 
 
@@ -116,10 +116,15 @@ def full_path(self) -> str:
     @staticmethod
     def _normalize_file_name(filename: str) -> str:
         supported_extensions = [".yaml"]
-        if not version.base_at_least("1.2"):
-            supported_extensions.append(".yml")
+        if version.base_at_least("1.2"):
             if filename.endswith(".yml"):
-                deprecation_warning(
+                raise UnsupportedExtensionException(
+                    ".yml files are not supported. Use .yaml extension for Hydra config files."
+                )
+        else:
+             supported_extensions.append(".yml")
+             if filename.endswith(".yml"):
+                 deprecation_warning(
                     "Support for .yml files is deprecated. Use .yaml extension for Hydra config files"
                 )
         if not any(filename.endswith(ext) for ext in supported_extensions):
diff --git a/tests/test_config_loader.py b/tests/test_config_loader.py
index ebf0088a19..ac128532cc 100644
--- a/tests/test_config_loader.py
+++ b/tests/test_config_loader.py
@@ -17,6 +17,7 @@
     ConfigCompositionException,
     HydraException,
     MissingConfigException,
+    UnsupportedExtensionException,
 )
 from hydra.test_utils.test_utils import chdir_hydra_root
 from hydra.types import RunMode
@@ -190,6 +191,12 @@ def test_load_yml_file(self, path: str, hydra_restore_singletons: Any) -> None:
         config_loader = ConfigLoaderImpl(
             config_search_path=create_config_search_path(path)
         )
+        with raises(UnsupportedExtensionException):
+            cfg = config_loader.load_configuration(
+                config_name="config.yml",
+                overrides=[],
+                run_mode=RunMode.RUN,
+            )
         version.setbase("1.1")
         with warns(
             UserWarning,