O2 customers affected after a technical fault left millions unable to get online could be in line for compensation.

The mobile network suffered disruption from 5am on Thursday, with full service not returning until early today.

Guidance from regulator Ofcom said that, depending on circumstances, it "may be appropriate" for mobile networks to offer some form of compensation for loss of service.

O2 said it would update customers later on Friday on how "we will make yesterday's data service issue up to them".

Customers wishing to complain should follow a company's formal complaints procedure - usually completing an online form or phoning the company directly.

If customers are not happy with how a complaint has been responded to, or it takes more than eight weeks to be resolved, a further complaint can be submitted to an independent Alternative Dispute Resolution (ADR) scheme.

In the case of O2, this is Ofcom-approved Ombudsman Services: Communications, which reviews a case and comes to an independent decision on it, usually within six weeks.

Sky Mobile, one of the operators affected by the outage as it uses O2 network, has already confirmed it will compensate customers with a day of free unlimited data.

The network said: "We're really sorry for the issues you experienced on Sky Mobile yesterday.

"We're giving you a day of free unlimited UK data this Saturday 8 Dec, so you can catch up on all the things you've missed. You don't need to do a thing."

Meanwhile, O2 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.

"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."

The company had earlier issued a joint apology with telecoms company Ericsson.

O2 UK chief executive Mark Evans said: "I want to let our customers know how sorry I am for the impact our network data issue has had on them.

"We fully appreciate it's been a poor experience and we are really sorry."

O2, which has more than 25 million UK customers, saw disruption to its network last for most of the day on Thursday.

Other mobile networks, including Sky, Tesco and Giffgaff, were also affected by the problem because their networks use O2 services.

Marielle Lindgren, chief executive of Ericsson UK and Ireland, said: "The faulty software that has caused these issues is being decommissioned."

She added: "Ericsson sincerely apologises to customers for the inconvenience caused."