diff --git a/aiogram/dispatcher/handler.py b/aiogram/dispatcher/handler.py
index da37fe3a..80562633 100644
--- a/aiogram/dispatcher/handler.py
+++ b/aiogram/dispatcher/handler.py
@@ -44,10 +44,6 @@ class Handler:
         self.handlers: List[Handler.HandlerObj] = []
         self.middleware_key = middleware_key
 
-    @property
-    def formatted_middleware_key(self):
-        return "pre_process_"+self.middleware_key
-
     def register(self, handler, filters=None, index=None):
         """
         Register callback
@@ -102,7 +98,7 @@ class Handler:
 
         if self.middleware_key:
             try:
-                await self.dispatcher.middleware.trigger(self.formatted_middleware_key, args + (data,))
+                await self.dispatcher.middleware.trigger(f"pre_process_{self.middleware_key}", args + (data,))
             except CancelHandler:  # Allow to cancel current event
                 return results
 
@@ -116,7 +112,7 @@ class Handler:
                     ctx_token = current_handler.set(handler_obj.handler)
                     try:
                         if self.middleware_key:
-                            await self.dispatcher.middleware.trigger(self.formatted_middleware_key, args + (data,))
+                            await self.dispatcher.middleware.trigger(f"process_{self.middleware_key}", args + (data,))
                         partial_data = _check_spec(handler_obj.spec, data)
                         response = await handler_obj.handler(*args, **partial_data)
                         if response is not None:
diff --git a/aiogram/dispatcher/middlewares.py b/aiogram/dispatcher/middlewares.py
index 6534a164..89b5f1dc 100644
--- a/aiogram/dispatcher/middlewares.py
+++ b/aiogram/dispatcher/middlewares.py
@@ -96,7 +96,7 @@ class BaseMiddleware:
         :param args:
         :return:
         """
-        handler_name = f"on_"+action
+        handler_name = 'on_' + action
         handler = getattr(self, handler_name, None)
         if not handler:
             return None