Tesco is challenging Asda’s positioning as the UK’s cheapest supermarket with a new campaign claiming Asda’s big green arrow ads are “misleading”.

The new Tesco adverts use items collected from Clubcard data on 220,000 shops on Friday 2 January to claim that Tesco was cheaper on 1.2 million shops while Asda was cheaper on 645,000. The retailer said the campaign showed “who really is the cheapest supermarket”.

Tesco said Asda’s long-running ad campaign, based on data from online price comparison website mysupermarket.com, was selective in its choice of items and did not represent typical shops. The ads included no own-label lines, despite own label making up half an average grocery shop, while featured 162 hair dyes and 64 dog treats.

“For years, retailers have made claims and counter- claims about who is the cheapest. But until now nobody has looked at what shoppers actually put in their baskets,” said Tesco commercial and marketing director Richard Brasher.

But Asda hit back. “Week in week out for the past 11 years, millions of shoppers know Asda is Britain’s lowest-priced supermarket as independently verified by The Grocer 33,” said a spokesman. “It’s no accident that when value is more important to people we’re seeing more customers than ever switching to shop at Asda.”

Asda said its ads were restricted to brands to ensure only identical products were compared, while Tesco’s ads used the “closest product match”.

The Grocer 33, which tracks both branded and own-label everyday items, has found Asda to be the cheapest retailer in the past six months. Asda was cheapest for 18 of the past 28 weeks, Morrisons was cheapest for six, Tesco for three and Sainsbury's for one. Asda’s average basket price was £54.88, against £55.80 at Morrisons and £56.76 at Tesco.

The retailer has also come out cheapest in each of the previous 11 years of The Grocer survey.