WHATSAPP has still to explain what caused a worldwide outage which left millions unable to use the service.
The breakdown left chat users in the UK, Europe, Asia and America unable to send and receive text messages, photos or videos on Wednesday.
The Facebook-owned messaging app, which has more than 1.2 billion users, started to experience problems from around 9pm. But by 11.20pm, many reported being able to access the service once again.
WhatsApp has not commented on the cause of the problem and only issued a statement acknowledging there was an issue, but did not explain any cause behind it.
"We are currently experiencing a service outage," it said on Wednesday night.
"Our team is working to restore service as soon as possible. We appreciate your patience."
Complaints posted on social media quickly made the service the top trend in the UK on Twitter. Many users reported being able to open the app, but not able to connect to its servers, making it impossible to send or receive messages.
While WhatsApp has claimed the issue has now been fixed, according to independent website DownDetector, a number of UK users were last night still experiencing issues with the app.
This is not the first time the app has experienced a breakdown with a major outage reported on New Year's Eve.
