Bank of Scotland (BoS) - part of the UK's Hbos banking group - is notifying more than 60,000 mortgage customers that a computer disc containing their personal information has been lost in the postal system.
BoS says the disc contained names, addresses and dates of birth and mortgage account numbers for 62,000 mortgage customers, but it did not contain bank account details, PINs, passwords or transaction data.
The bank had sent the disc via Royal Mail to a credit reference agency more than two weeks ago, but the data never arrived.
In a statement BoS says "there is no suggestion that the disc was stolen" and it would appear to have been "mislaid in the post".
Shane O'Riordain, GM, group communications, BoS, says: "We have taken immediate steps to protect our customers and make sure this does not happen again. We have written to all affected customers today to apologise and to reassure them that the risk of fraud is very low."
BoS says it is "almost impossible" that any financial fraud could be committed with the data held on the disc, but it is offering affected customers free registration with CIFAS, the UK's fraud prevention service. This facility provides additional protection against the misuse of personal information when applying for credit and other products and services.
The lost disc has been reported to both the Financial Services Authority (FSA) and the Information Commissioner.
The latest incident comes just two months after Halifax - another Hbos subsidiary - reported that the mortgage details of 13,000 customers had been stolen from an employee's car. Paper documents containing the confidential information - which included names, addresses, mortgage account numbers and balance information - were contained in a briefcase that was stolen from the employee's locked car.
Furthermore, in a separate incident earlier this year Hbos said it was investigating how a customer who requested a copy of her bank statement ended up being sent the confidential details of 75,000 other account holders. Stephanie McLaughlin, 22, had asked Hbos for one copy of her bank statement but received five packages containing 2500 sheets featuring other customers' names, sort codes and account details instead.