The company also apologized for the inconvenience.
The issue affected nodes in the core networks of customers using two software versions of the Serving GPRS Support Node - Mobility Management Entity (SGSN-MME), Ericsson said, adding that it had taken immediate action to enable service restoration. Work is now ongoing to fix the fault and customers are being encouraged to use wi-fi where possible.
"While it's positive that voice calls are still up and running, without a projected timeframe for a fix, this is likely a worrying situation for a large proportion of O2's some 32 million United Kingdom customers". O2 says technical teams are working "extremely hard to resolve this".
They said in a statement: "The network issue we are now experiencing remains a top priority for us and we are working as quickly as possible to fix the issues".
British customers reported not being able to use mobile data to access the internet and the operator's network on Thursday after disruption began at about 5am.
He said they were caused by a mechanical problem in Softbank's 4G telecommunications systems.
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Customers with the mobile phone company O2 have been left without service today after the network went down..
In an online statement, O2 said: "We are aware our customers are unable to use data this morning".
O2 has 32 million customers on its network while SoftBank hosts 40 million mobile customers.
"The network was recovered to the normal operation by adapting the older version of the software to all packet switching machines". The O2 network also powers services for the Sky, Tesco, Giffgaff, and Lycamobile networks, which have an additional seven million users total.
Computing will update this story as soon more news comes in.
Ericsson said in a press release that most services had been restored. Here is what business professionals and mobile users need to know about 5G networks.