Spanish banking giant Santander has sent bank statements for up to 35,000 customers to the wrong addresses.
The group said a printing glitch led to customers being sent the first page of their own bank statement but pages two and three of another customer's.
The statements included customers' names, account numbers and details of transactions they had carried out, although Santander insisted this was not enough information to enable someone to commit fraud on their account.
Despite this, it has written to all affected customers informing them of what has happened.
It has also warned all of its branch and call centre staff not to reveal account information to anyone without taking them through the full security procedure, which includes people having to give extra information, such as their mother's maiden name.
The group said it had also informed City watchdog the Financial Services Authority about the problem.