O2 has issued an apology after millions of its customers were unable to get online on Thursday due to a technical fault.

The mobile operator said its 4G data network has now been restored after a day of disruption for smartphone users, but stressed that its technical teams will continue to monitor service performance over the next few days.

The company will be carrying out a review with systems and equipment provider Ericsson to "understand fully what happened".

O2 issued an apology early Friday morning and promised to work with Ericsson "through the night".

"We fully appreciate it’s been a poor experience and we are really sorry," said Mark Evans, CEO of Telefonica (O2) UK.

Marielle Lindgren, CEO Ericsson UK & 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."