Backed by a £40m advertising campaign, Nectar, the new loyalty card service from Sainsbury's, Debenhams, BP and Barclaycard, was launched last Monday. The programme hoped to attract 50% of UK households in its first year.
But it has now taken the registration part of its Web site down following the security breach, which allowed a customer to see another person's private information.
A spokeswoman for Loyalty Management UK, the company that is running the scheme, said: "We have taken the registration part of the site down because of continued problems with the encrypted areas. The decision has been taken to update that part of the site, rather than to disappoint customers with slow service."
In a statement on the Nectar site, Loyalty Management UK recommended that customers wishing to register should call the Nectar helpline to request a paper form to complete and post in.
The security breach was not the only IT problem that Nectar faced. A spokewoman for the company admitted that thousands of users were unable access the www.nectar.com site because LMUK's Sun servers could not cope with the high demand.
The spokeswoman said LMUK's IT team would continue to work over the weekend to increase the server capacity and to update the encrypted area of the site.