A rather red-faced Which? Online admitted this morning it had inadvertently released the names and credit card details of thousands of its customers on its website.
The names and addresses, along with various other details, of 2,700 people who had purchased TaxCalc software from one of the recommended Which? sites were accessible for most of yesterday.
Thousands of customers who were affected will be contacted by the company and urged to cancel their credit card details immediately.
The alarm was raised by the Times newspaper late last night and the Consumers Association immediately closed the site.
"As soon as we were alerted to the flaw, we removed all personal and financial details from the site," said Kim Lavely, deputy editor of Which? Online. "This is a serious flaw in the TaxCalc site and we are very concerned about it."
Vision Ideas Consulting, the company which created the software website, was unavailable for comment.