"We apologise for any inconvenience", the firm said, after problems were first reported at around 5.30am.
The network outage has also affected Transport For London, which uses the O2 network for its electronic timetable displays at bus stops.
TFL said: "The company that updates the data to our network of Countdown Systems is now experiencing difficulties in providing this service".
It doesn't end there, because some major MVNO (virtual networks) that piggyback O2 signals are experiencing the same disruption. But data connections are broken. Woke up this morning with no mobile data at all and I have turned mobile data off and also powered my device down and still no mobile data? Here is what business professionals and mobile users need to know about 5G networks. "Down? Really?! @O2 appalling coverage all the time anyway, can't wait to switch out my service provider".
Wisconsin Republicans to Vote on Curbing Newly Elected Democrats' Power
Another element of the bill would make it more hard for Evers to alter some GOP-backed laws, including Wisconsin's voter ID law. Vos and Fitzgerald were not present to defend their proposals or listen to the hours of impassioned opposition.
Virat Kohli | Kuldeep Yadav | India vs Australia 2018-19
I can't describe what it is, not in front of the media, but he's made some adjustments which have helped him. I really enjoy the city, I don't know what the connection is, but I just feel really good here.
Netanyahu Says 'Border Tunnels' Operation to 'Continue as Long as Necessary'
A Lebanese army source said the situation was calm on its side of the border, as did United Nations peacekeepers operating there. Hezbollah, meanwhile, has also been heavily engaged in Syria, where it is battling alongside government forces.
Many have been unable to access the Internet after reporting issues with 4G and data services.
The company said one of their third party suppliers has identfied a software issue in their system. We believe other mobile operators around the world are also affected.
O2 said it will be sharing updates on its website when the service changes.
Ericsson, whose central user database caused O2's 2012 25-hour mega-outage, told The Reg: "We are aware of the issue and are working together with our customers to solve it as soon as possible". Work is now ongoing to fix the fault and customers are being encouraged to use wi-fi where possible.