UK retailer Morrisons has said it is considering complaining to the advertising watchdog about Tesco‘s Price Promise, which it says is “misleading”.

The grocer said today (5 April) the scheme, launched last month, does not correctly compare the price of Tesco products with Morrisons products. It added the promise is “misleading” customers into thinking Tesco will always compensate them if they shop at Tesco but that Morrisons is cheaper.

The Price Promise, which compares own-label and branded products with those of its rivals, is a till-based system that gives consumers a voucher of up to GBP10 if their “comparable shopping” would have been cheaper at their rivals.

Morrisons, however, claims it has checked the price comparisons and found Tesco is unable to make a comparison in many cases and lists Morrisons prices as ‘not available’. It said Tesco may only check Morrisons prices manually through mystery shopping exercises. Morrisons does not yet sell food online.

“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,” said Crawford Davidson, customer director for Morrisons.

“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.”

Morrisons said it has contacted Tesco about its concerns but did not disclose whether it had received a response.

A spokesperson for Tesco, however, said: “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.

“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.”

Price Promise was launched last month and prompted withering responses from Tesco’s other main rivals, Asda and Sainsbury’s.

“With the Asda Price 10% Guarantee our promise is to beat prices, not just match them,” Asda said.

Sainsbury said the promotion was just Tesco “playing catch up”. The UK’s third-largest retailer introduced its own Brand Match comparison scheme in October 2011.