Barclays has opened a probe into possible theft of about 27,000 customer files with personal and financial information, following a report published by the Daily Mail.
The probe is the latest in a series of blows that the British bank faced in the recent times.
Although reasons for the theft are yet to be known, the missing files could include information related to income, savings, mortgages, health issues, insurance policies, passport and national insurance from 2008 or earlier.
Barclays in a statement said that the incident appears to be criminal action and assured to cooperate with the authorities on on pursuing the perpetrator.
"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," Barclays added.
In July 2013, the bank agreed to pay $453m to the US and UK regulators for manipulating the Libor rate.