The incident, involving a Nationwide Building Society employee, happened in August, but the details have only just been released.

The computer holds identity information about customers but it does not have pin numbers or passwords needed to access their accounts.

The building society's chief executive, Philip Williamson, told Sky News:

"The reason we didn't let customers know about this incident earlier was that we didn't wish to create undue anxiety.

"There is no chance of any customer suffering from any financial loss on their accounts as a result of this."

The Financial Services Authority (FSA) is investigating.