An investigation by the consumer group found the schemes run by Asda, Sainsbury's and Tesco, in which customers are given coupons if their shopping would have been cheaper elsewhere, differ greatly, making them of little real worth to shoppers.
Secret shoppers visited the supermarkets to find that, in the majority of cases, each claimed to be cheaper than its rivals.
Which? executive director Richard Lloyd said: "Supermarket price-matching schemes can save you money but we believe they should be taken with a pinch of salt because they are difficult to compare."
Meanwhile, an advert for the Sainsbury's Brand Match campaign has been banned for "misleadingly" implying that consumers did not need to shop around to benefit fully from deals at rivals Tesco and Asda.