It said: "This allowed hundreds of people from both our technical teams and our third-party supplier's teams to focus on fixing the problem, helping us to restore data services as quickly as we can which will also help reduce the high network demand we're now seeing for voice calls".
"This remains our priority and we apologise to our customers for any inconvenience".
Following their initial announcement it confirmed the cause of issues by posting on Twitter: "Outage Update: One of our 3rd party suppliers has identified a software issue in their system".
The outage is thought to be affecting about 32 million in the UK.
Millions of people have been having problems with their mobile phones this morning, after the O2 network went down. Transport for London said: "The company that updates the data to our network of Countdown Systems is now experiencing difficulties in providing this service".
Companies such as Tesco Mobile, Sky Mobile and GiffGaff, that share the O2 network, are also experiencing problems. "This issue is being investigated".
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As LTE networks become increasingly saturated, mobile network operators are planning for the 5G future. But data connections are broken.
O2 has not said what is causing the fault on Thursday.
A statement on their website said engineers are investigating issues related to data problems.
Another said: "This is ridiculous apparently it's the whole country I can't just rely on wifi".
The spokesperson added: "Voice calls are working OK".
The company have so far not said anything about whether they would be giving customers compensation for the outage.