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.
– Barclays spokeswoman
Our initial investigations suggest this is isolated to customers linked to our Barclays Financial Planning business which we ceased operating as a service in 2011.
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.
Protecting our customers' data is a top priority and we take this issue extremely seriously.
This appears to be criminal action and we will co-operate with the authorities on pursuing the perpetrator.