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Mobile operator O2 is understood to be seeking millions in damages from supplier Ericsson after last week's day-long data network collapse.

The total bill could be up to £100m, according to The Telegraph.

O2 smartphone users were unable to use their mobile phone data last Thursday. Ericsson blamed expired software certification for the problem.

Both firms have apologised for the issue and O2 has already set out how it plans to compensate customers.

Customers with a monthly subscription will be refunded the cost of two days' service by the end of January.

Pay As You Go customers will get 10% extra when they top up their phone in the new year or 10% off when they buy data for mobile broadband devices.

O2 said voice calls were not affected by the problem, but some customers said they could not make calls or send texts either.

The mobile phone operator is owned by Spain's Telefonica and has the UK's second-largest mobile network after EE, which is part of BT. It is the company that bills customers, so it holds the responsibility for compensation.

O2 has 25 million users and also provides services for the Sky, Tesco, Giffgaff and Lycamobile networks, which have another seven million users.

Services such as bus timetable information were also affected by last week's outage, while many businesses also faced disruption.

Telefonica's UK chief executive Mark Evans told the BBC last week it planned a "full audit" of the problem.

"What we will now do is a full audit, a thorough audit, across both organisations to ensure whatever steps can be taken will be taken to provide the continuous service that our customers expect and deserve."

Ericsson said last week that "an initial root cause analysis" had indicated that the "main issue was an expired certificate in the software versions installed with these customers".

"The faulty software that has caused these issues is being decommissioned," Marielle Lindgren, chief executive of Ericsson UK & Ireland, said at the time.