Spanish banking giant Santander has sent over 22,000 UK bank statements to the wrong addresses as the result of an error at the bank's printers.
Santander owns UK financial services firms Abbey National, Alliance and Leicester and Bradford & Bingley.
The bank said it has identified 22,600 affected customers, although reports put the figure at 35,000. It has informed the Financial Services Authority.
A spokesman asked customers who have received incorrect information, "which will be will be page 2 and 3 etc", to destroy it. "We will be resending statements. If customers are worried they should visit a branch or give us a call," he added.
A statement from the bank said, "Due to a technical error at our printers a number of current account statements dated 18 December have printed incorrectly. We are sorry for any inconvenience this has caused and have taken immediate steps to correct this.
We take the security of customer and account information extremely seriously so any incident of this nature is treated with the highest priority. We want to reassure customers that the risk of fraud on their account has not been increased because of this error and that this is a one-off incident."
The Information Commissioner's Office (ICO) is investigating.
An ICO spokesperson said: "We have recently been informed of a data breach involving Santander. We will be making enquiries into the circumstances of the apparent breach of the Data Protection Act before deciding what action, if any, needs to be taken.
"Under the Data Protection Act, organisations that process personal information have an obligation to keep it secure; therefore, it is a matter of concern if information such as account details have been incorrectly provided to the wrong recipient. Banks risk losing the confidence and trust of their customers if they fail to safeguard personal information. It is therefore in the interests of both the bank and the customer that this information is handled in line with the requirements of the Act."