A Briton has informed authorities that a computer he bought through eBay for $142 contained data on several million credit card customers. The incident has sparked an investigation in the U.S. and the U.K.

The buyer, 56-year-old IT manager Andrew Chapman of Oxford, England, contacted authorities when he found on the computer's hard drive the account numbers, passwords, cell phone numbers and signatures of credit card customers of the Royal Bank of Scotland (RBS), its subsidiary NatWest and American Express.

A third party apparently sold online the computer belonging to MailSource UK, a data processing firm under Graphic Data, without permission. The latter is an archiving firm that stores financial information for the three banks.

"The IT equipment that appeared on eBay was neither planned nor instructed by the company to be disposed," a spokesman of MailSource UK told BBC News.

"Clearly such details should never have been included in the hard drive of the computer offered for sale on eBay," said a spokesman of the U.S.-run auction and shopping website.

The Information Commissioner's Office will investigate the incident to determine who is liable for violating a law protecting personal information. A spokesman for American Express said it is trying to determine which accounts are affected.

RBS is talking with Graphic Data to find out how the computer ended up on eBay.