Ericsson is blaming software for outages that left more than 30 million O2 UK customers without data services for most of yesterday.
The UK operator's network went down about 5am on Thursday, bringing chaos to its data services and sporadic outages for voice. Japanese operator Softbank was also left stricken by the problems although it was able to reboot its network after four hours.
Ericsson specifically blamed issues in certain core network nodes that used two specific types of Serving GPRS Support Node - Mobility Management Entity software. It said an initial assessment suggested an expired certificate in this software caused the issue.
It said the software would be decommissioned but somewhat insensitively described an outage that affected tens of millions across two continents as "disturbances for a limited number of customers".
In the UK, O2's LTE network was restored this morning. A spokesperson said: "Our technical teams will continue to monitor service performance closely over the next few days to ensure we remain stable.
"A review will be carried out with Ericsson to understand fully what happened.
"We’d like to thank our customers for their patience during the loss of service on Thursday 6 December and we’re sorry for any impact the issue may have caused."
Speaking yesterday, O2 UK CEO Marc Evans said: "We fully appreciate it’s been a poor experience and we are really sorry."
According to the latest figures from network testers Rootmetrics, O2 was deemed to have the worst network in the UK and also scored fourth for reliability.
The company said O2 needed to focus on expanding its LTE footprint beyond the UK's key towns and cities.
It recommended it deploy the spectrum it won in the country's spectrum auction this year to improve its services.