Three has gone down, leaving users with no service.
Vast numbers of users reported issues with receiving messages and other problems.
“We’re aware that a small number of customers may be experiencing issues with our network this morning,” the company wrote on Twitter.
“Our engineers are working hard to fix this as soon as possible. We’re so sorry if you’ve been impacted by this.”
Three was launched 20 years ago and has just over 10 million customers in the UK, making it the fourth-biggest mobile network in the company. It says that it is responsible for 29 per cent of the UK’s mobile data traffic.
But it is set to undergo a major shake-up as it merges with Vodafone, in a deal set to be completed by the end of next year. Customers will be moved to the new company, which is still unnamed.