An investigation is under way after confidential files relating to customers of Barclays Bank were allegedly stolen and sold on to rogue City traders.
The Mail on Sunday said highly sensitive information, including customers' earnings, savings, mortgages, health issues and insurance policies, ended up in the hands of unscrupulous brokers.
The leak was exposed by an anonymous whistle-blower who passed the newspaper a memory stick containing files on 2,000 of the bank's customers, it said.
He claimed it was a sample from a stolen database of up to 27,000 files, which he said could be sold by shady salesmen for up to £50 per file.
Each report is about 20 pages long, and among the victims are doctors, businessmen, scientists, a musician and a cleaner, the newspaper said.
All the customers had sought financial advice from the bank, and passed on their details during meetings with an adviser.
Select traders were given the 'Barclays leads', the newspaper said, and from December 2012 to September last year a number of victims were persuaded to buy rare earth metals that did not exist, it is claimed.
A Barclays spokeswoman said: "We are grateful to the Mail on Sunday for bringing this to our attention and we contacted the Information Commissioner and other regulators on Friday as soon as we were made aware.
"Our initial investigations suggest this is isolated to customers linked to our Barclays Financial Planning business which we ceased operating as a service in 2011.
"We will take all necessary steps to contact and advise those customers as soon as possible so that they can also ensure the safety of their personal data.
"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."
A source for the bank said it had not yet verified that the memory stick contained 2,000 files, or how much information was on the files, and there was no evidence yet that 27,000 were involved.
A spokesman for the Information Commissioner's Office 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."
City of London police had no information on the case.