Mobile network O2 says its services have been restored after a technical fault left millions of customers unable to get online.

The company said it would be closely monitoring data services over the coming days and promised to carry out a review to understand what went wrong.

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.

On Thursday evening, O2 said 3G data service had started returning and was expected to be fully restored by 9.30pm, while the company reported at 3.30am on Friday that the 4G network had been restored.

"Our technical teams will continue to monitor service performance closely over the next few days to ensure we remain stable," a spokesman said.

The Telegraph understands the outage is being treated as a technical fault with O2's supplier Ericsson, not a cyber security incident. "A review will be carried out with Ericsson to understand fully what happened," O2 said.

The company said it was working to fix the problem, although gave no timeline of when the service would be back online. The company said users should connect to Wi-Fi when possible if they needed access to data services.