Subscribers to mobile network 3 have been hit by a network outage this morning, leaving many without internet, though voice and text services remain unaffected.
"Some of our customers are unable to access the internet. We are working to restore their data services as soon as possible," said a spokesman for the company. "We apologise to all those affected for any inconvenience."
Of the network's nine million customers in the UK it was impossible to say how many were affected by today's "intermittent" problem.
The ThreeUKSupport Twitter feed claimed the problem was being "worked on as a priority" and apologised "for any inconvenience".
A notice on Three's website repeated this message.
In customer satisfaction surveys on mobile networks carried out by Ofcom Three was ranked the second most satisfactory service, with 02 ranked number one and Vodafone receiving the most complaints.
The Three spokesman said late this morning that the service had been restored in London and it was hoped that the entire network would be back to normal this afternoon.
He said: "Some of our customers have had problems accessing the internet, we're restoring those services right now.
"We're happy to say internet access across London is back up and running and the whole network should be back to normal this afternoon, if not sooner. We are sorry for any interruption. Our engineers are working hard to make sure our customers can keep on internetting."