The chief executive of the company that owns the O2 mobile phone network has apologised to customers over today's outage.
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 UK
Now 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.