According to a report yesterday, highly sensitive information, including customers' earnings, savings, mortgages, health issues and insurance policies, had ended up in the hands of unscrupulous brokers.
The leak was exposed by an anonymous whistle-blower who passed on a memory stick containing files on 2000 of the bank's customers, the report 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 was about 20 pages long, and among the victims were apparently doctors, businessmen, a musician and a cleaner.
Select traders were given the "Barclays leads", the report said, and from December 2012 to September last year a number of victims were allegedly persuaded to buy metals that did not exist.
The whistle-blower estimates up to 1000 people could have been "scammed".
A Barclays spokeswoman said the bank had contacted the Information Commissioner as soon as it was made aware of the claims.
She added: "Protecting our customers' data is a top priority and we take this issue extremely seriously. 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."