Asda has agreed to change its price promotions such as multi-buy deals after the competition watchdog singled it out during an investigation into supermarket pricing tactics.
The Competition and Markets Authority (CMA) announced it had asked a number of supermarkets to review their promotions in response to a super-complaint by Which? accusing supermarkets of ripping off consumers with misleading and confusing prices.
The CMA said it "has had particular engagement with Asda in relation to specific areas of concern", and the grocer had "given a commitment to the CMA that it will change the way it operates 'was/now' and multi-buy deals".
The CMA said it had been assured by Asda that "was/now" prices will give shoppers a meaningful comparison and the grocer will not advertise the "now" price for longer than the original, while multi-buy offers will be better value than a single product before the deal.
Asda has also promised not to immediately follow multi-buy offers with "was/now" promotions, so it will be easier for shoppers to tell what is a good offer.
Which? lodged its complaint with the CMA in April last year, claiming retailers were creating the illusion of savings through the use of multi-buys, shrinking products and baffling sales offers.
The CMA reported in July that it had found that supermarkets were misleading customers with confusing pricing promotions that could be against the law.
It found "areas of poor practice that could confuse or mislead shoppers" and it was taking measures to help make sure grocers complied with the rules.
It said it had since met with a number of supermarkets and expected them to review their pricing and promotional practices and make any necessary changes to ensure consumers can be confident they are getting a good deal.
The CMA's executive director of enforcement, Michael Grenfell, said: "The CMA's examination of the market, following the super-complaint, found that supermarkets generally take compliance seriously, but there were some promotional practices that could mislead shoppers.
"We welcome the commitment we have received from Asda as well as the engagement from other supermarkets, and expect them all to ensure that their practices are not misleading and that shoppers are better informed and able to choose the products that most suit their needs."
CMA chief executive Alex Chisholm said: "This draws the CMA's follow-up work to the super-complaint to an end.
"In addition to our work with Asda and the other retailers, we are pleased to see that our recommendations concerning the key legislation and guidance in this area are being taken forward."
Asda chief executive Andy Clarke said: "We are consistently recognised as the UK's lowest price, full range supermarket, offering prices that are at least 10% less than those of our rivals.
"Asda has won the Grocer 33, the most credible pricing measure in our industry, for the last 18 years and we're pleased that the CMA has today recognised that we take pricing compliance seriously."
Which? executive director Richard Lloyd said: "Following our super-complaint last year, we are pleased to see the CMA investigation has resulted in Asda taking action to stop misleading special offers. Asda has been found breaking the rules and now must immediately clean up their act.
"Our super-complaint and actions taken by the authorities should serve as a clear warning to all retailers. If they try to pull the wool over consumers' eyes, they will not get away with it. Retailers must get their house in order."
Mr Clarke later added: "All supermarkets were asked to review their pricing practices and make any necessary changes. The CMA has asked for a commitment from Asda on our promotional pricing rules and we were happy to provide this.
"It's important that customers know that the CMA did not make any findings againstAsda and it hasn't questioned our commitment to everyday low prices."