Both the FSA and Nationwide have refused to say exactly what was on the laptop, which was taken in a burglary in August.
However, the building society confirmed it held customer information, but maintained that this did not include pin numbers, passwords or transaction details.
According to Alan Oliver, head of external affairs at Nationwide, the device contained "limited customer information for market research purposes".
Nationwide insists that victims of identity fraud would not suffer financial loss, as the building society has a policy of reimbursing money stolen. A spokesperson said: "There has been no loss of money, and no chance of any customers suffering financial loss. If they are the innocent victim of fraud they will not lose out. The information on its own cannot be used for identity fraud."
Nevertheless, it's claimed that criminals could combine the personal data stolen with other customer information and use it for identity fraud.
It's reported that as a result of the theft Nationwide has clamped down on customer data being carried on staff laptops and has begun writing to all of its 11 million consumers outlining the security measures they need to take.
Although the building society was keen to play down the severity of its security lapse, the banking industry regulator - FSA - is probing the incident.
"We're continuing to discuss with Nationwide the incidence of a loss of data," said a FSA spokesperson. "Our principle concern is to minimise the risk to consumers."
Gary Clark, VP of EMEA, SafeNet commented: "Once again, a laptop containing confidential consumer data is stolen, and the need for stricter security for mobile devices is highlighted once more."
"‘Random thefts' or loss of laptops and other physical assets inevitably occur, however, if access to the data on these stolen items is protected, this information will remain protected and out of the reach of criminals."
Stolen laptop triggers security investigation
By Fiona Raisbeck on Nov 14, 2006 4:01PM