Bank and credit card customers across Northern Ireland were today waiting to see if their personal details were among the one million discovered on a computer sold on eBay for Å35.
The huge data loss, which has been described as one of the worst in Britain, involved information on customers from the Royal Bank of Scotland (RBS), NatWest and American Express.
Over a million addresses, mobile phone numbers, bank account numbers, sort codes, credit card numbers, mothers' maiden names and signatures were found on the computer's hard drive. The Ulster Bank, a subsidiary of the RBS, was today investigating if its customers were affected by the security lapse.
A spokesman for the bank told the Belfast Telegraph: "They are looking into it."
The huge breach of highly sensitive information has led to IT security experts warning had it got into the wrong hands it could have allowed a thief to go on "an enormous shopping spree".
The massive blunder was discovered by an IT manager who checked the hard-drive of a second hand computer he bought through internet auction site eBay.
Andrew Chapman, from Oxford, said he was left completely shocked by what he found.
"I couldn't believe it," he told the Daily Telegraph.
"In front of me was reams of extremely confidential information about thousands and thousands of people."
The computer was sold for Å35.88 earlier this month by an ex-employee of archive firm Graphic Data, which stores the records of some of Britain's biggest financial organisations at its headquarters in Essex.
Before selling it he did not erase the internal hard drive.
Jenny Thomas, a spokeswoman for eBay, said that an item such as this should never have been sold on eBay and that the company is working with Graphic Data to investigate the sale.
Some of the details included thousands of applications for credit cards to NatWest.
There are also more than 1,300 credit card balance transfer requests received by American Express.
It has also emerged a second computer has gone missing.
IT security expert Adam Laurie said: "This is appalling. This information is worth millions."
NatWest/RBS said: "Graphic Data has confirmed to us that one of their machines appears to have been inappropriately sold on via a third party.
"RBS and NatWest take data protection extremely seriously and have very strict procedures to ensure the security of information at all times.