Thousands of British people are being urged to cancel their credit cards after details were made freely available on a website.
The British Consumers' Association says about 2,700 people are affected.
The error came about after people bought the TaxCalc software, via the internet using a credit card as payment.
Following the discovery of the flaw, the site has been shut down.
Ms Kim Lavely, deputy director of the Consumers' Association, said: "This is a serious flaw in the security of the TaxCalc site and we are very concerned about it.
"As soon as we were alerted to the flaw, we removed all personal and financial details from the TaxCalc site.
"We have already commissioned an independent security expert to conduct a thorough audit of the security of this site."
PA