One of Britain's biggest mobile phone companies has admitted to a major cyber-security breach which could put the personal data of millions of customers at risk.

Three Mobile admitted that hackers have successfully accessed its customer upgrade database after using an employee login.

Sources familiar with the incident told the Telegraph that the private information of two thirds of the company's nine million customers could be at risk.

The company confirmed the breach on Thursday evening but declined to say whether customers' data was stolen or how many have been affected.

Three said that the data accessed included names, phone numbers, addresses and dates of birth, but added that it did not include financial information.

It comes after Talk Talk, the phone and broadband company, admitted in October last year that the private details of 157,000 customers had been hacked.