Britain's supermarkets are facing calls for a competition inquiry after Which? accused firms of ripping off shoppers with misleading and confusing prices.
In its super-complaint to the Competition and Markets Authority (CMA), the consumer group said retailers were creating the illusion of savings through the use of multi-buys, shrinking products and baffling sales offers.
Which? said 40% of groceries are sold on promotion so consumers could be collectively losing out to the tune of hundreds of millions of pounds if only a small proportion of offers are misleading.
It added that it was virtually impossible for people to know if they are getting a fair deal, particularly when prices vary frequently or when consumers are in a rush and may be buying numerous items of relatively low value.
The group is one of a handful of bodies with the power to make a super-complaint on behalf of consumers about poorly functioning markets to the CMA, which must now respond within 90 days..
Promotions have become more widely used in the industry over the last 12 months as major supermarkets have cut prices to fight losses in market share to discounters such as Aldi and Lidl.
The consumer body said over the last seven years it has catalogued a range of misleading pricing practices and has passed a report of its findings to the CMA.
Which? executive director Richard Lloyd said: "Despite Which? repeatedly exposing misleading and confusing pricing tactics, and calling for voluntary change by the retailers, these dodgy offers remain on numerous supermarket shelves.
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"Shoppers think they're getting a bargain but in reality it's impossible for any consumer to know if they're genuinely getting a fair deal."
Which? said an example of a misleading multi-buy offer came when Asda increased the price of a Chicago Town Four Cheese Pizza Two-Pack from £1.50 to £2 last year as it went onto multi-buy at two for £3. It went back to £1.50 after the offer ended.
The consumer body also criticised an example of 'was/now' pricing earlier this year when Heston from Waitrose Acacia Honey & Ginger Hot Cross Buns were advertised at £1.50 for just 12 days before going on offer at "£1.12 was £1.50" for 26 days. In this instance the item was available at the lower price for longer.
Mr Lloyd said: "We're saying enough is enough and using one of the most powerful legal weapons in our armoury to act on behalf of consumers by launching a super-complaint to the regulator. We want an end to misleading pricing tactics and for all retailers to use fair pricing that people can trust."
This is only the sixth time Which? has used its super-complaint power since it was granted the right in 2002.
It last issued a super-compliant in 2011 when it asked the Office of Fair Trading (OFT) to investigate excessive credit and debit card surcharges. The OFT upheld its complaint.