Investigations have been launched after a report that thousands of confidential files with details of Barclays Bank customers had been stolen and sold.
The Mail on Sunday said a database of information on up to 27,000 customers was sold to "unscrupulous brokers".
It reportedly covered money and health matters and attitude to risk, national insurance and passport numbers.
Barclays said it had contacted regulators and would help authorities in "pursuing the perpetrator".
The newspaper said the leak was exposed by an anonymous whistleblower who handed it a memory stick containing personal data belonging to 2000 Barclays customers.
He said data belonging to a further 25,000 was on sale for £50 per customer.
The report said the data included earnings, savings, mortgages, insurance policies, health issues and attitude to risk, as well as national insurance and passport numbers, and was "worth millions on the black market because it allowed unsuspecting individuals to be targeted in investment scams".
The BBC reports the customers in question had originally contacted the bank seeking financial advice from Barclays Financial Planning, which was shut down in 2011.
The Information Commissioner's Office said it would be working with the newspaper and police who were seeking more details.