O2 networks have been down since 1pm yesterday.
O2 confirmed on its Twitter feed yesterday afternoon that it was "having a problem with Pay Monthly internet access [which] will affect MMS too," leading to many customers unable to send emails and MMS.
Phone calls were unaffected, unless using VoIP, and pay as you go customers also seemed to avoid the outage.
The last Twitter update from the provider was 18 hours ago saying: "We're working to fix it ASAP," but some customers are still complaining of problems this morning.
O2 has exclusivity for some of the most web-friendly phones, including the Apple iPhone - although that could soon change.
IT PRO contacted O2 who have just confirmed the problem and apologised.
A spokesperson from the company said: "We have identified the cause of the fault, which relates to the allocation of IP addresses needed to establish a data connection."
"We have managed to restore data services to the vast majority of customers and are doing everything we can to ensure the few remaining customers are back online as soon as possible."
