Frustrated customers from London to as far a field as North Wales and the Highlands reported they were completely cut off from around noon yesterday and were still disconnected today. The network, whose slogan is "We're better connected", has still not issued a timetable for a solution.
O2 claimed engineers had been working through the night to fix the problem.
In an update at 5.40am this morning, O2 conceded the problems were still not fixed. "We're continuing to drive the recovery of this issue with our support teams and vendors. We're working to restore service as quickly as possible. Unfortunately we're unable to confirm any timescales for service restoration at this time," the company said. "We'll let you know as soon as we can. We apologise once again for any inconvenience this incident is causing."
Subscribers to networks GiffGaff and Tesco Mobile - who make use of O2's infrastructure - have also been affected.
Some users this morning reported that service had been restored, while others said they had now been affected by the problem.
A spokesman for O2, which boasts of its "award-winning network" and 200 engineers "dedicated to network performance", said last night that he had no information on how many customers were affected, what was causing the outage or when it would be fixed. The firm, part of Spain’s Telefonica group, is Britain’s second-largest mobile operator, with 22 million customers.
“We are currently seeing a problem on our network affecting some of our customers. Those customers affected will have difficulty making or receiving calls, sending texts or using data,” the spokesman said.
“Our engineers are dealing with the problem as a priority and we hope to restore full service as soon as possible."
The fault is apparently related with how mobile phones register with O2's network, rather than geography. meaning that affected users can be standing next to other O2 customers who are able to use the service as normal.
The major failure is O2’s second in two weeks. In June, thousands of customers were left unable to send texts for a whole day.
Many were frustrated by the lack of information released by O2 today. The firm’s Twitter account was deluged with complaints but only issued apologies and promises the unidentified fault would be restored “as soon as possible”.
Stars and customers took to Twitter, with Gareth Malone and Huw Edwards among the most high-profile users affected.
Conservative MP Rob Halfon, a keen technology enthusiast and leading light in analysing Google's WiFi snooping scandal, even asked "Have O2 lost it?"
Former Home Secretary Jacqui Smith also said that she had been unable to appear by phone on London radio station LBC because of the problems.
Asked if it would compensate customers in their next bill, O2 said that "right now, our priority is getting our customers back up and running".
Helplines were meanwhile jammed and O2’s network status web page was intermittently unavailable, as servers struggled to cope with the crush of customers seeking details. Several Twitter commentators compared the failure to the recent IT systems collapse at Natwest and RBS, although that affected customers for a week.
Michael Allen, director of IT service management at technology performance company Compuware said, "Unfortunately, these problems will only continue to increase unless organisations take a fundamentally different approach to the way they manage the performance of the IT systems we rely on to go about our day to day lives. O2’s ability to deliver a service to customers will rely on hundreds of different components, systems and applications working in harmony. This can make preventing these types of service disruptions difficult as well as finding the root cause time consuming."
