diff --git a/telethon/utils.py b/telethon/utils.py
index fb558e41..ef9dc0a6 100644
--- a/telethon/utils.py
+++ b/telethon/utils.py
@@ -44,7 +44,7 @@ USERNAME_RE = re.compile(
 #
 # See https://telegram.org/blog/inline-bots#how-does-it-work
 VALID_USERNAME_RE = re.compile(
-    r'^([a-z][\w\d]{3,30}[a-z\d]'
+    r'^([a-z]((?!__)[\w\d]){3,30}[a-z\d]'
     r'|gif|vid|pic|bing|wiki|imdb|bold|vote|like|coub|ya)$',
     re.IGNORECASE
 )