Morrisons has been told it must be clearer about its promise to match the prices of competitors, including Aldi and Lidl.
The supermarket's Match & More scheme, which is the only one of the big four to match the prices of the German discounters, has been pulled up by the advertising watchdog for not clearly comparing products after a complaint by Aldi.
The TV and press ads about the loyalty scheme garnered several complaints from the supermarket as well as members of the public. Four of the five complaints were dismissed, but a fifth was upheld.
The Advertising Standards Authority found the ad did not give enough information for customers to be able to verify the claims it made in terms of comparing prices and warned Morrisons it should give details of the products and prices being compared and how points gained from the loyalty scheme are calculated.
The loyalty scheme, which is less than a year old and was launched under ousted chief executive Dalton Philip in a bid to reverse the supermarket's slump, has been criticised by analysts as over-complicated.
New chief David Potts promised to look at the scheme as part of his wide-ranging review of the business.