O2 says that it does not know when full service will be restored to millions of customers who can not access data services on their smartphones.
The mobile network said it was working "all-around" with Ericsson to solve technical problems.
The Swedish company, which manufactures mobile network equipment, confirmed that it was a problem with its software.
Services such as bus timetable information even companies have been interrupted.
O2 has 25 million customers and also provides services for the Sky, Tesco, Giffgaff and Lycamobile networks, which have an additional seven million users.
"The network problem we are currently experiencing remains a priority for us and we are working as quickly as possible to solve the problem," said O2.
Its CEO, Mark Evans, tweeted to "reassure our customers that we are doing everything we can to solve the problem with our network."
Ericsson said there were network outages for customers in multiple countries.
President Börje Ekholm said: "The faulty software that caused these problems is being decommissioned and we apologize not only with our customers but also with their customers, we are committed to ensuring that our customers can limit impact and restore their services as soon as possible. "
The network services on most of the affected networks were restored on Thursday, he added. "We are working closely with the remaining customers who are still having problems."
The Japanese network Y! Mobile, owned by Softbank, was also affected by the interruptions.
Difficulties in the UK were reported for the first time at 05:30 on Thursday
Network down: how people are facing without a signal
The Spanish Telefonica owns O2, which has the second mobile network of the United Kingdom after EE, which is now part of BT.
The company said that voice calls are not affected by the problem, but some O2 customers say they can not make calls or send messages either.
The interruption is having knock-on effects for other services using the O2 network, including the Transport for London electronic timetable service at bus stops, which has stopped working.
William Webb, a technical consultant and former director of OFOM, said it was unusual to spend so much time resolving the problem. problem: "It's probably some sort of software update that one of the network providers issued that for some reason turned out to be incompatible or failed ̵
T om Morrod at Market research firm IHS Markit said data are increasingly important to consumers, with half of UK mobile users preferring Internet connectivity over calls and texts.
"In addition to consumer hardship and associated frustration, having a large network out of order creates productivity challenges for businesses, many companies will have commuters or field workers who have lost work time today," he said. .
An O2 client, Allison Rose-Mannall from Norwich, is an insulin-dependent diabetic who relies on his cell phone and is not able to reach a fixed network.
"I'm disabled … I'm in a wheelchair," he told the BBC. "So I do not have any data, but no call means I can not contact anyone if I have a fall or if I need something."
Lynsey Greaves runs a company in Doncaster providing home visits to the elderly and vulnerable. Its 130 employees rely on all O2 phones to access rotations, times, names and addresses. From 5:30 he called the extra staff office to provide information for each visit on the phone.
"There are nine of us who are trying to solve it now," he said. "It was a nightmare."
Luke Stagg is trying to run a plumbing business through his phone, but can not contact customers or use his satnav.
"It's all a day wasted," he says. "I will try to recover my losses, especially as a business customer."
But some people actually accepted the forced interruption of the screen.
"Remember the not too distant past, where people were not wandering like zombies, glued to social media?" says customer O2 David Flanders . "Remember to have a chat with the stranger near you on the train or on the bus?
" I certainly do not intend to lose sleep by losing my data for a while. " "
Can I ask for compensation?  O2 customers will be able to claim the out-of-pocket costs of being without a phone, according to the consumer expert Helen Dewdney.
Ms. Dewdney, who writes a blog called The Complaining Cow, said users should be able to claim under the Consumer Rights Act 2015, which states that services should be carried out with reasonable skill and care.
This means that customers could requesting a refund for what they would pay on a contract for the time they were without the use of the phone and could also claim a consequent loss due to a breach of the contract, for example, if they had incurred bank charges because they were not in able to transfer their money or the cost of having to use a public telephone.  This must be a real loss that can be proved d with evidence.Then, a taxi driver could they can prove that they have lost rates due to closure, but other workers who try to claim a lost day's work would have difficulty.
Ms. Dewdney suggested that phone users calculate losses and wrote to O2 with evidence. They should declare what they want as compensation and mention the Consumer Rights Act 2015.
If they thought the answer was unsatisfactory, they could be referred to Ombudsman Services: Communications of which O2 is a member. The customer should ask for a deadlock letter or wait eight weeks from the start of the claim.