The UK's supermarkets are to face an investigation from the regulator over claims of "confusing and misleading" pricing tactics and special offers.
Consumer body Which? has used its legal powers to make a "super-complaint" to the Competition and Markets Authority (CMA) over "dodgy multi-buys and baffling sales offers" that "creates the illusion of savings that doesn't exist."
It also identified pack sizes that shrink while the price stays the same and seasonal offers where, for example, Easter or Christmas products are sold at a higher price months, only to appear "on sale" during the festive period.
It said consumers - who spend £115 billion on groceries annually - could be collectively losing out on hundreds of millions of pounds a year due to such practices and that it only took the step to contact the CMA after it repeatedly raised the issue with the industry during the last seven years, but nothing had changed.
Numerous dodgy offers
In its complaint to the CMA, who now must respond within 90 days, Which? highlighted a number of issues, including Asda increasing the price of a Chicago Town Pizza Two Pack from £1.50 to £2 when it went onto multi-buy sale at two for £3 before reverting back to £1.50 when the offer had ended.
Another example included a Nestle Kit Kat Chunky Collection Giant Easter Egg which was advertised at £7.49 for just 10 days in January at Ocado but was then sold on "offer" at £5 for 51 days.
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. 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.
"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."
Which? is one of five consumer bodies that have the power to launch a "super-complaint." It last issued such a complaint in June 2011, when it asked the Office of Fair Trading to investigate excessive credit and debit card surcharges.