Messaging service WhatsApp will no longer work on millions of smartphones from 1 February.
Android and iPhone devices which only support outdated operating systems will no longer be able to run the Facebook-owned app. Certain devices, such as iPhone 4s, which only support iOS 7, will no longer be compatible with the app.
WhatsApp said the move was necessary in order to protect the security of its users.
Smartphones using Android 2.3.7 and older, and iPhone iOS 8 or older, are those affected by the update.
The operating systems that WhatsApp is dropping support for are legacy operating systems, which are no longer updated or installed on new devices.
Most users will simply be able to update their operating systems in order to continue using the messaging service.