The press and television adverts involved a product price comparison with rival Bradford-based supermarket chain Morrisons, which complained to the Advertising Standards Authority (ASA) claiming the adverts were misleading.

The adverts said 2,955 branded products at Asda were cheaper than at Morrisons and the prices had been checked independently by sources including mySupermarket.co.uk

The ASA found the ads did not comply with case law from the European Court of Justice because they did not set out how consumers or competitors could check the information for themselves.

It ruled future ads must show people how to verify the prices for themselves by, for example, including a link to a website.

The ASA found the adverts were not misleading on two further points and found the overall comparison was based on a fair selection of products.

An Asda spokesman said: "This ad was aired in February 2008 and we fell down on a technicality.

"We have since changed the wording for ads where we compare our prices with Morrisons by stating that customers can contact Asda head office for a list of products.

"This wording will remain as long as Morrisons fail to offer home shopping which would allow customers to check prices themselves."