Once you've confirmed, your account will be deleted, and all your data will be removed from Telegram's servers. This includes your chats, groups, and contacts - so, make sure you're really ready to say goodbye! You won't be able to recover your account, so don't come crying to me if you change your mind later.
Now, I know what you're thinking: "What about my secret chats?" Don't worry, those will be deleted too, so you can rest assured that your private conversations will remain private. It's like they never existed in the first place!
How to delete your Telegram account the right way
