Mobile telecommunications carrier T-Mobile informed the UK’s data protection watchdog, the Information Commissioner’s Office (ICO), that its employees sold customers’ account details to competitors without their permission, it was revealed yesterday.

The ICO issued a statement revealing that a mobile services provider had contacted it alleging that “information was being sold on to the service provider’s competitors whose agents were using the material to cold call customers prior to contract expiry dates to offer them an alternative contract. The service provider has alleged that many thousands of customer account details have been unlawfully obtained.”

T-Mobile later revealed that it was the company in question. "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),” said a T-Mobile spokesperson. "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."

The case was presented by the ICO as proof that it needs the power to impose custodial sentences on individuals who break data protection laws. It “shows that a deterrent custodial sentence is required to stop the trade in unlawful personal information,” the body said.