Scandal-hit U.K. bank Barclays has launched an investigation after a newspaper reported that the personal details of 27,000 customers had been stolen and sold.
Confidential customer details on their earnings and health as well as passport details had ended up for sale, "The Mail on Sunday" reported after it had been contacted by a whistleblower.
(Read more: Barclays profit falls, cooperating in forex probe)
The "Mail On Sunday" report added that the customer data, which also detailed individuals' attitude to risk, had been sold to "rogue City traders" and was "worth millions on the black market because it allowed unsuspecting individuals to be targeted in investment scams".
In a statement released by Barclays Sunday, the bank thanked the newspaper for bringing the matter to its attention and added that it had contacted the U.K. Information Commission and banking authorities on Friday as soon as it had become aware of the theft.
(Read more: Barclays CEO turns down bonus for 2013)
Barclays added that initial investigations suggested the files were linked to the Barclays Financial Planning business which closed in 2011 and the stolen data dated back to 2008 or earlier.
"This appears to be criminal action and we will co-operate with the authorities on pursuing the perpetrator," Barclays said.
"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."
The data leak is the latest blow for the British bank after a series of mis-selling and interest rate-fixing scandals.
Last December the bank was fined $3.75 million by U.S. regulators over an alleged decade-long failure to properly keep electronic records, emails and instant messages.
And last week, the Barclays chief executive Antony Jenkins announced he had turned down his 2013 bonus, saying it would be inappropriate given the bank's hefty bill to pay for past problems.