Barclays has said it has launched an investigation into the theft of customer data from its Barclays Finanical Planning unit.
The Mail on Sunday revealed an anonymous whistleblower had provided the newspaper with a memory stick containing the personal data of 2,000 customers, before claiming information on a further 25,000 is available.
Files are thought to contain passport and national insurance information, as well as financial and health data.
The newspaper claimed that the data is worth millions on the black market, as unsuspecting individuals could be targeted in investment scams.
Barclays said it had contacted regulators as soon as it had been made aware of the reports.
''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," a Barclays spokesman said.
The stolen files date from 2008 and contain data of individuals who contacted the bank seeking financial advice from Barclays Financial Planning.
By Asim Shah