WhatsApp is down for thousands of people across the UK, Europe and Asia.
Users started bemoaning not being able to send and receive messages on Sunday morning.
More than 500million people use the app, which is owned by Facebook, every day.
According to Down Detector London is the worst affected area in Britain, with Paris, large swathes of Belgium, Germany and the Netherlands facing outages.
Areas of Asia and the Middle East, including Dubai, India and Malaysia are also suffering problems.