Thousands of T-Mobile customers' private details were sold on to rival firms in a major data selling scandal, it has been revealed.

A spokesman for the company confirmed it alerted the data protection watchdog after it discovered contract information was being passed on "without our knowledge".

The Information Commissioner, who is investigating the breach of data protection rules, said thousands of mobile phone owners had their details sold on for "substantial sums".

A spokesman for the company said the situation was "deeply regrettable" and suggested it was an industry-wide problem.

The company had been asked to keep the problem a secret to avoid prejudicing a criminal prosecution.

The spokesman said yesterday: "T-Mobile takes the protection of customer information seriously.

"When it became apparent that contract renewal information was being passed on to third parties without our knowledge, we alerted the Information Commissioner's Office (ICO). Working together, we identified the source of the breach, which led to the ICO conducting an extensive investigation which we believe we will lead to a prosecution.

"While it is deeply regrettable that customer information has been misappropriated in this way, we have proactively supported the ICO to help stamp out what is a problem for the whole industry."

He added: "We had been asked before today to keep all information on this case strictly confidential so as to avoid prejudice to the investigation and prosecution.

"We were therefore surprised at the way in which these statements were made to the BBC today."