WhatsApp went down early on Tuesday, with users across the UK and around the world unable to send or receive messages.
Users of the application, owned and run by Meta, began reporting issues around 8am on Tuesday.
Many people complained on social media that they could not send or receive messages – the app has around two billion users across the world.
More than 12,000 reports were posted within the first half hour, according to real-time monitoring website DownDetector.
WhatsApp confirmed it was aware, and resolved the issue at around 10am.
After opening the app on Tuesday morning, users found that – while they still had access to their conversations – the app was failing to deliver new messages or send any that they attempted to transmit.
A persistent message was showing for many users at the top of the app, saying it was “connecting” to the server, but then appeared to fail to do so.
In a statement, a WhatsApp spokesperson said: “We’re aware that some people are currently having trouble sending messages and we’re working to restore WhatsApp for everyone as quickly as possible.”