Millions of O2 mobile phone users have been left unable to access the internet because of a technical fault, with services unlikely to return until Friday morning at the earliest.
Many customers also reported being unable to make or receive voice calls, which O2 said was due to the company receiving many complaints and queries via phone, which meant they have been "experiencing a high demand on our network".
The mobile network said it had set up an urgent investigation after customers reported being unable to get online or use mobile data from around 4.45am of Thursday.
Late on Thursday night O2 said its 3G data service had been restored, and that work was still underway on restoring its 4G service.
ITV News Consumer Editor Chris Choi said that around 25 million mobile phone users had been affected.
Other mobile networks, including Tesco and Giffgaff, also experienced problems because they use O2 services.
Buses around London were also affected as countdown screens at stops, which rely on O2 services, subsequently went down.
What has O2 said the problem is?
Responding to the blackout, O2 said that one of its third party suppliers, Ericsson, was experiencing software issues.
Both companies have apologised to customers
"We are aware our customers are unable to use data this morning," O2 said on Twitter.
"Our technical teams are working on the issue with high priority. We are really sorry and working as hard and as fast as we can to fix this."
An update on Thursday evening from CEO Mark Evans said in a statement: "I want to let our customers know how sorry I am for the impact our network data issue has had on them, and reassure them that our teams, together with Ericsson, are doing everything we can.
"We will continue to work with Ericsson, through the night, who have assured us that a full service will be restored for customers by the morning.
"We fully appreciate it's been a poor experience and we are really sorry."
A spokesperson for Ericsson said they were "aware of the issue and are working together with our customers to solve it as soon as possible".
Marielle Lindgren, the CEO of Ericsson UK and Ireland, added: "The cause of today's network issue is in certain nodes in the core network resulting in network disturbances for a limited number of customers across the world, including in the UK.
"We have been working hard on resolving the UK data issue since early this morning. The faulty software that has caused these issues is being decommissioned.
"Our priority is to restore full data services on the network by tomorrow morning.Ericsson sincerely apologises to customers for the inconvenience caused."
What have Tesco and Giffgaff said?
Giffgaff and Tesco acknowledged there had been interruptions to their networks, but said service would return overnight into Friday morning.
Giffgaff said this had impacted 2G, 3G and 4G services as well as voice services.
Tesco Mobile said that they were aware customers were unable to use their data.
Lycamobile and Sky also use O2, but it was unclear if these services had been affected.
How does this issue affect London buses?
Tracker systems which rely on the O2 network have also been affected by the outage, meaning countdown screens at London bus stops have stopped working.
Screens which normally display the predicted arrival time of buses across the capital are now blank.
A TfL spokesperson said: "We're sorry that customers are unable to use our Countdown screens at bus stops for live travel information.
"This is a result of a nationwide O2 data outage. We are working with our service provider to resolve this as soon as possible."
How have people been responding?
Thousands of people took to Twitter to establish whether the data issues were limited to individual phones or part of a wider problem.
The issue began trending under the hashtag #o2down, with people either complaining about the blackout or joking that the lack of internet had left them with no choice but to interact with other humans.