Barclays Bank sought to reassure customers that it had "taken every practical measure" to ensure their personal and financial details "remain as safe and secure as possible" following The Mail on Sunday's claims of a data breach.
The bank said it contacted the Information Commissioner and other regulators on Friday, when it was made aware of the report.
Thousands of confidential files relating to customers of Barclays Bank were allegedly stolen and sold on to rogue City traders, according to a report in The Mail on Sunday.
The paper said that a whistleblower exposed the leak, which allegedly involves a database of up to 27,000 files containing information including customers' earnings, savings, mortgages, health issues and insurance policies.
Rogue City traders used the 'Barclays leads' between December 2012 and September last year to persuade a number of victims to buy rare earth metals that did not exist, it is claimed.
Each report is about 20 pages long, and among the victims are doctors, businessmen, scientists, a musician and a cleaner, the newspaper said.