Fraudsters had access to the personal details of 30,000 Barclays customers for up to seven years, it has been revealed.
Private information on jobs, salaries and debts was found on a memory stick by chance in a police raid, and it is feared that multiple copies of the data may have been made.
The Daily Mail reports that the treasure trove of information also included names, dates of birth and addresses, and that victims have been offered £250 in compensation for the breach.
A Barclays spokesman said: "This is not a new theft of data from Barclays. Every indication is that the data here was part of the same theft of data that was reported last year, relating to data stolen in 2008. It is simply a separate USB data stick that was not received at that point in time and was recently discovered by the police.
"As with the theft last year, the details on the recently discovered USB data stick belong to a group of customers linked to the Barclays Financial Planning business which ceased operating in 2011. The data concerned was from 2008 or earlier."
He added: "We have proactively contacted the affected customers to apologise, as well as to offer them enhanced fraud protection and monitoring.
"We have also proactively offered customers compensation for the inconvenience this will have no doubt caused them.
"We are asking each customer to call if they feel their individual circumstances warrant different compensation, especially if they believe that they had a fraudulent event at any point since this data was stolen.
"We have also proactively reviewed all data that we hold to see if we can see any sign of suspicious activity and will continue to do that.
"Protecting our customers' data is our highest priority, and we take this issue extremely seriously."