Personal bank account details of up to one million Natwest and Royal Bank of Scotland (RBS) customers have been found on a computer that was sold for £35 on eBAY, the online auction site.

The information on the computer is believed to include names, addresses, mobile phone numbers, bank account numbers, sort codes, credit card numbers, mothers' maiden names and even signatures.

The computer, which was "inappropriately" sold by an employee at Graphic Data, an Essex-based company, also contained sensitive balance transfer information about American Express credit card customers.

An IT expert who bought the computer discovered the data in the hard drive of the computer.

A spokeswoman for RBS, which owns Natwest, today confirmed that the private details of customers had been compromised.

"Graphic Data has confirmed to us that one of their machines has been inappropriately sold on via a third party," RBS said in a statement.

"As a result, historical data relating to credit card applications of some of our customers and data from other banks were removed. We take this issue extremely seriously and are working to resolve this regrettable loss of Graphic Data as a matter of urgency."

American Express was unavailable for comment.

It is unclear if the customers whose details were exposed will be forced to change their credit cards and bank accounts.

The companies involved in the blunder could be forced to pay a fine. Last year, the Financial Services Authority fined Nationwide £980,000 after it lost a laptop containing customer information.

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