The personal details of up to 27,000 Barclays customers have been stolen and sold on the black market.
The data is said to include financial information such as earnings and savings, as well as passport and National Insurance numbers and medical history of customers of the now defunct Barclays Financial Planning.
The personal details now at risk come from customer files dating as far back as 2008.
The theft emerged after a Sunday newspaper was handed a memory stick containing the confidential data by a whistleblower.
The newspaper informed independent watchdog the Information Commissioner's Office (ICO) of the security breach and it will now investigate the matter alongside police and the bank.
Barclays could be fined £500,000 for putting its customers' personal details at risk.
Customers to be contacted
While no dedicated customer helpline has been set up, Barclays said it will take "all necessary steps to contact and advise those customers as soon as possible".
It added: "Protecting our customers' data is a top priority and we take this issue extremely seriously. This appears to be criminal action and we will co-operate with the authorities on pursuing the perpetrator.
"We would like to reassure all of our customers that we have taken every practical measure to ensure that personal and financial details remain as safe and secure as possible."
An ICO spokesperson said: "It's crucial that people's personal information is properly looked after. We'll be working with the Mail on Sunday this week to get further details of what has happened here, as well as working with the police."