Customers have been suffering problems since around 4am and many are still unable to make calls, send texts or use their mobile data.
02 network down across UKNow in a tweet, Mark Evans, CEO of O2 owner Telefonica UK has said: "I want to reassure our customers that we are doing everything we can to fix the issue with our network and say how sorry I am to everyone affected.
"My teams are working really hard with Ericsson to find a swift resolution. Stay updated: https://status.o2.co.uk/."
The company said one of their third party suppliers had identfied a software issue in their system. Work is currently ongoing to fix the fault and customers are being encouraged to use wi-fi where possible.
O2 network down across the UK - can you claim compensation?Companies that share O2's network – such as GiffGaff, Sky Mobile and Tesco Mobile – are also being hit by the problems.