More than 40 million credit card users have been put at risk of fraud after hackers gained access to a database storing card users’ details.

The security breach took place at a payment processing centre in Atlanta, and affects all brands of card, including 22 million Visa accounts and 14 million operated by MasterCard.

A MasterCard spokeswoman, Jessica Antle, said that only about 68,000 of its card holders are at "higher levels of risk", and that those customers should closely examine their credit or debit card accounts. Individual customers will not have to pay for any fraudulent transactions, which will be covered by the credit card companies.

The breach was reportedly caused by a hacker who gained access to the databases of CardSystems Solutions and installed a program that searched for details of transactions. However, Ms Antle said that customers would not be at risk of identity theft as only limited personal details are stored on credit cards.

The incident appears to be the largest yet involving financial data in a series of security breaches affecting valuable consumer data at major financial institutions. Two months ago, 180,000 HSBC customers were warned to replace their cards after a retailer’s card scanner was found to have retained transaction details.

The FBI is now investigating the fraud, which was discovered when CardSystems’ security systems detected an unusual pattern of transactions.