A computer bought on eBay Inc.'s online auction site held the personal information of at least a million customers of Royal Bank of Scotland Group PLC, its Natwest unit, and American Express Co., according to reports Tuesday.

The ZDNet.co.uk blog said details included names, addresses, bank account numbers, telephone numbers, and customer signatures. The BBC said the details also included mothers' maiden names. The data-archiving firm Graphic Data had used the computer to store the details for RBS and its Natwest unit. Graphic Data said it had not authorized the computer's disposal and is investigating how it was put up for auction. eBay also said it is investigating.

The man who bought the computer reported the data breach when he discovered it.