Police and Barclays are investigating after confidential files of thousands of the bank's customers were allegedly stolen and sold on.

The Mail on Sunday said an anonymous whistleblower passed it a memory stick containing the personal information of 2,000 Barclays customers, while claiming a further 25,000 were also available (see our ID Fraud Protection guide for tips on protecting yourself).

Barclays says it will contact all 2,000 affected customers this week.

The newspaper said the files were from 2008 and were each about 20 pages long. They contained details of customers' earnings, savings, mortgages, and insurance policies – plus passport and national insurance numbers – which the whistleblower said could be sold on for up to £50 per file.

The bank says all affected customers had sought financial advice from the bank's Financial Planning division, which was shut down in 2011.

Files used to 'scam' victims

The Mail on Sunday also said the data contained individuals' attitudes to to risk, reportedly making it worth millions on the black market because it allows unsuspecting individuals to be targeted in investment scams.

From December 2012 to September last year, a number of victims were persuaded to buy rare earth metals that did not exist, it is claimed.

The paper's whistleblower estimated up to 1,000 people could have been 'scammed'.

However, Barclays says: "No evidence has been provided at this stage that the files have been used for illegitimate purpose, simply that they could potentially have been."

How do I know if my details have been lost?

Barclays told MoneySavingExpert.com it will contact the 2,000 affected customers "by the end of this week". But if you're worried or need advice now, you can call the Barclays helpline on 03332 027 509, which it has set up specifically for victims of this data loss.

A Barclays spokesman says: "For people who are previous customers of Barclays Financial Planning, we have a specialist team in place that will contact them by phone in the next week if they are impacted.

"For people who were not previous customers of Barclays Financial Planning, we can advise that from the data we have been provided by the Mail on Sunday, we don't think they are impacted.

"If any evidence comes to light to suggest otherwise, we will contact them."

The bank added that it wants to "understand any detriment they may have suffered" but could not confirm if those affected will be compensated.

Regulator investigating

Barclays says it has reported the data loss to the Information Commissioner's Office, a data protection regulator that can impose fines of up to £500,000 when organisations fail to protect private data.

A spokesman for the Information Commissioner's Office says: "It's crucial that people's personal information is properly looked after. We'll be working with the Mail on Sunday this week to get further details of what has happened here, as well as working with the police."

The Financial Conduct Authority has also been told about the issue.

Additional reporting by the Press Association.