Morrisons has questioned Tesco over the accuracy of its latest Price Promise advertising ahead of a possible complaint to the Advertising Standards Authority.

Tesco’s Price Promise compares a shoppers’ basket with prices of products from other supermarkets – both branded and own label. Morrisons said in a statement today it had checked these price comparisons and found that Tesco was unable to make a comparison in many cases.

Morrisons also said it had conducted a shopping exercise based on The Grocer 33 mystery shopping survey. This exercise included five products that Tesco had been unable to compare. The result, Morrisons said, was that its basket was £1.05 cheaper than Tesco’s, despite Price Promise claiming that Tesco was cheaper.

“If Tesco can’t accurately collect prices of its rivals and make trustworthy comparisons, then it should not be making this so-called promise” - Crawford Davidson, Morrisons

Crawford Davidson, customer director at Morrisons, said: “We are complaining because Tesco’s Price Promise provides a false assurance that it will compensate customers when Morrisons is cheaper, which it mostly is.

“Customers expect Tesco to do this properly.  If Tesco can’t accurately collect prices of its rivals and make trustworthy comparisons, then it should not be making this so-called promise.”

Tesco first introduced Price Promise in Northern Irish stores in November, ahead of a national rollout last month.

“By making sure we give them back the difference if they could have got a better deal elsewhere, Price Promise gives our customers confidence that they won’t lose out by shopping at Tesco,” said a Tesco spokesperson.

“We use an independent agency to check prices of branded and own-label products at other retailers – online daily for Asda and Sainsbury’s, and since they don’t have an online grocery service, twice a week at Morrisons stores. The basis for our comparisons is made clear on the Price Promise website.”

Under new ASA rules, rival businesses must demonstrate they have attempted to resolve their issues directly before making a complaint official. The watchdog brought the rule in last year to stem the rising tide of tit-for-tat complaints between rivals.

Morrisons said it was now considering a complaint to the ASA. In the mean time, it said it had contacted Tesco about its concerns.

Sainsbury’s CEO Justin King has also questioned the validity of the comparisons. He argued price comparisons on own label did not reflect differences in quality.

“You can be sure we will be pointing out to Tesco where we believe their own-label products are being unfairly compared to ours,” he said last month.