The personal details of more than a million high- street bank customers have been found on the hard drive of a computer sold on eBay for £35.
The information on customers of NatWest, the Royal Bank of Scotland and American Express included mobile phone numbers, bank account numbers, mothers' maiden names and signatures.
The computer had belonged to Graphic Data, a company which stores information for financial organisations, and was used at the firm's archive in Shoeburyness, Essex. It was sold on the internet auction site by an ex-employee this month.
The buyer, Andrew Chapman, an IT manager from Oxford, found the bank details on the its hard drive.
The Liberal Democrat spokesman Tom Brake said: "This is yet another example of a seemingly trusted organisation appearing to be sloppy with people's personal information ... People are entitled to wonder why they are constantly being told about the importance of protecting personal information when large organisations don't seem to follow the same rules themselves."
American Express and NatWest/RBS said they were investigating, but would need to establish how many clients were involved before deciding on a course of action. Graphic Data said it was trying to recover the computer.