Asda has been hit by a website glitch affecting its Price Guarantee scheme that has meant customers have claimed for food vouchers worth thousands of pounds.

Shoppers can earn money off vouchers with the supermarket if they paid more for their weekly shop at Asda than at any of its large rivals. Under the scheme, Asda will also add 10% to the value of the vouchers.

But a problem in the system has meant it has been undervaluing some rivals’ products, resulting in Asda distributing more vouchers than it should have, according to The Daily Telegraph.

Shoppers have been taking advantage of the error and tips have been shared on internet forums about how to exploit the scheme. Some people claim they have been making up to £40 per shop, and one said they had made more than £8,500 from Asda in recent weeks because of the problem.

Asda is attempting to combat the issue and since Sunday has began capping the vouchers to £15 per shop. A spokesman said: ” A very small number of our savviest customers have been able to beat the system and claim a few, larger than normal, money-off vouchers.

“Customers can still claim up to £100 in Asda Price Guarantee vouchers per calendar month, or up to 10 vouchers in a month but not exceeding £100. This is monitored through the website, and colleagues will not be required to enforce this in store.”

THe maintained that the cap will not impact the “vast majority of customers”.

“We will continue to offer the most transparent, robust price comparison in the market and maintain our guarantee to be 10% cheaper than our rivals on a comparable basket of groceries, or we will refund the difference,” he said.