Consumers should take supermarket price match schemes with "a pinch of salt" because each retailer calculates the cost of a basket differently, according to the watchdog Which?.
An investigation by the consumer group found the schemes run by Asda, Sainsbury's and Tesco 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? analysed the till receipts of 19 Asda shops, 20 Tesco stores and 20 Sainsbury's outlets, checking the price of each basket with the supermarket's own online price match.
Asda was the cheapest on the most occasions (17 out of 19) according to its "Price Guarantee", Sainsbury's was cheaper than Asda and Tesco for 10 of the visits and joint cheapest for another two, according to its "Brand Match", and Tesco was cheaper than Asda and Sainsbury's for 10 of the 20 visits according to its "Price Promise".
A recent Which? poll found that just 30% of shoppers trust supermarkets to charge a fair price for food.
Almost three-quarters of those polled (71%) said price was one of the most important factors when shopping for food, suggesting that price match schemes can be an attractive incentive.
However, the 59 shopping trips analysed by Which? resulted in an average discount voucher of just £1.45.
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.
"At a time when consumers are facing a squeeze on their household incomes, we want all the supermarkets to do whatever they can to help consumers find the best deal."
Populus surveyed 2,028 adults, of whom 1,969 were grocery shoppers, online between June 28 and 30.