Bank of Scotland (BoS) has apologised after mortgage details belonging to 62,000 customers were lost in the post.
About 62,000 customers are thought to have been affected
The bank has admitted that the mortgage information, sent via Royal Mail more than a fortnight ago, has disappeared.
The alarm was raised after a computer disc containing the details of the accounts failed to reach a credit reference agency.
BoS said the package was more likely to have been lost than stolen and that the risk of fraud was "almost impossible".
Shane O'Riordain, the bank's communication's manager said moves had now been made to ensure the same thing does not happen again.
He said: "We apologise to our customers for any inconvenience or upset this incident may have caused.
"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."
The information on the lost disc contained names, addresses, dates of birth and mortgage account numbers for customers.
The bank insists the disc did not contain any bank account details or PIN numbers and passwords.
The company have also said that no customer would be left out of pocket in the "unlikely event of fraudulent activity".