Tesco has slammed Mysupermarket.com for using traffic-light labelling to flag up the health credentials of grocery products - when it supports the rival Guideline Daily Amounts.

The price comparison website has introduced a new feature which, in addition to price, compares levels of fat, salt, sugar and calories in products on sale at major online grocery retailers.

It uses the same traffic light system endorsed by the Food Standards Agency, which compiled the nutrition information for the site. So when shoppers look up Tesco products, they will see nutrient levels colour-coded as red, amber and green.

Instore, however, Tesco's own-label products carry GDA labels.

Tesco said the move would confuse consumers. "We don't think this helps consumers and we were not consulted about it," said a Tesco spokesman. "Our opinion of traffic lights remains the same. They don't help a consumer make an informed choice. All they say is 'don't do that'. We all know it's bad to eat a cream cake but GDAs show how it can be integrated into a balanced diet. Without GDAs shoppers are not better off."

Mysupermarket.com director Johnny Stern defended the decision to use the traffic lights.

"Our customers had been asking us for health information for a long time and, as it's a complicated science, we approached the FSA to help us. Its traffic-light labelling seemed a good place to start since it is simple to understand. It is very visual and when it comes to a website it has to be seen."

Stern admitted Mysupermarket.com did not consult the retailers about the introduction of the new tool. "We are independent and only consult our customers - and in this case, the FSA. We believe this is what consumers want."

However, he said he had not ruled out the introduction of the GDA system.

"There are pros and cons to both systems, and we are looking at putting GDAs on as well."