The messaging platform WhatsApp has gone down for users across the UK and around the world.
Users of the platform, owned by Meta, began reporting issues just before 08:00 BST on Tuesday.
More than 12,000 reports were posted within half an hour, according to the service status website Down Detector.
Many people have complained on social media that they cannot send or receive messages. WhatsApp has about two billion active users globally.
It is one of the most popular messaging platforms in the UK.
Many users who tried to use the app on Tuesday morning were faced with a "connecting" message.
A spokesperson for Meta 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."
The outage is likely to cause significant disruption. The app, which offers end-to-end encryption for messages, is a popular method of group communication for social, work and parenting activities.
It has even been used, controversially, by government ministers.
According to internet monitoring organisation NetBlocks the outage is affecting users both in the UK and around the world.
The outage appears to have global reach, with BBC journalists in Australia and Ukraine reporting problems.