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.
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.
More top news
A medical examiner concluded that an accidental overdose had caused Tom Petty to suffer a fatal heart attack at his Malibu home in October.
One year on from his inauguration, Donald Trump's supporters are even more passionate about him and his cause than before he entered office.
Support for new parents has faced deep cuts in recent years - and some social workers warn it could lead to a rise in forced adoption.