Millions of phones will no longer be able to use WhatsApp as the chat app stops supporting older operating systems.
From Saturday, users on older devices will be blocked from creating new accounts or verifying existing accounts.
Facebook, the app's parent company, said the change was needed to ensure user security.
"Because we no longer actively develop for these operating systems, some features might stop functioning at any time," it said in a blog post.
The social media giant recommends updating to the latest software in order to continue using WhatsApp.
Smartphones running Android version 2.3.7 and older will lose support, as will any iPhone using iOS 8 and older.
Every year WhatsApp removes support for older devices which it says "don't offer the capabilities we need to expand our app's features in the future".
It said the decision was difficult but necessary.
"This was a tough decision for us to make, but the right one in order to give people better ways to keep in touch with friends, family, and loved ones."