WhatsApp went down for users across the UK this morning, with tens of thousands of people reporting being unable to send or receive messages.
Downdetector, which tracks outages, said more than 69,100 reports of problems had been submitted at 8.28am, with issues first detected just before 8am.
However, the number of users that experienced issues could be much higher due to the way the website collects data.
By around 9.30am, that figure had dropped to 29,917 - and just before 10am users were noticing that services were back up and running as usual.
"We know people had trouble sending messages on WhatsApp today. We've fixed the issue and apologise for any inconvenience," a spokesperson for the platform's parent company, Meta, said.
WhatsApp users from around the world went on social media platforms, such as Twitter, to complain that the app was not working.
The hashtag #whatsappdown was trending on Twitter, with more than 70,000 tweets and hundreds of memes flooding the internet.
Downdetector also found more than 11,000 users in India, and 19,000 in Singapore, had reported experiencing issues.
Users found that while they could open the app and access their conversations, it was failing to deliver new messages or successfully send any.
A persistent message appeared for many users at the top of the app, saying it is "connecting" to the server, but then failed to do so.
Some of the most concerned users were likely to be Conservative MPs, with the platform widely used by backbenchers, cabinet ministers and aides to discuss public messaging.
A cabinet reshuffle will be taking place today, so many MPs hopeful of landing a ministerial job were likely without a vital way of sounding out advisers on their chances of being appointed for the time the service was down.
The app has previously been identified as a tool of choice for MPs plotting against their leader, and Boris Johnson is said to have often been sent summaries of key government information via the platform while he was prime minister.
WhatsApp is one of the most popular messaging platforms in the UK and around the world, and is estimated to have more than two billion active users globally.