Retail giant Tesco has challenged online supermarket Ocado through the Advertising Standards Authority (ASA) over misleading price claims in a number of its adverts.

The national press advert by Ocado features the headline: "Tesco prices. Waitrose quality. Ocado delivery. (We thought you could do with some good news today.)"

Tesco, however has challenged whether the price match claim, which states: "We're matching Tesco prices on all household brands", are "misleading", as its own monitoring has shown that Ocado is "more expensive" than Tesco for 601 products in a sample of 3,811.

Separately, an email featuring the subject heading: "Ocado matches Tesco prices on branded goods from today", including products such as; Heinz Tomato Ketchup, marmite and Kellogg's Cornflakes, has also been questioned by Tesco.

In response, Ocado said it had reviewed the data submitted by Tesco and found that a small number of the product descriptions "did not match the Ocado product descriptions" and that Tesco had "incorrectly compared non-identical products".

The online retailer also claimed that since the price match policy was launched in March 2008, customers had identified only 41 products as having been "inadvertently omitted" from the price match.

The ASA upheld Tesco's claim and said that Ocado's methods of obtaining Tesco prices could mean that prices are out of date within a day.

"We told Ocado not to make the price match claim again until they had addressed the factors which had caused them to fail to price match a large number of items when the price match policy was launched," the ASA said.

In March 2008 Ocado was forced to hit back at an earlier threat by Tesco to complain to the ASA about one of its ad campaigns, saying its larger rival had a reputation for "responding aggressively".

The threat however, followed a claim by Ocado that it would match Tesco's prices on "all household brands" in a programme that would see prices cut on 3,500 lines.