Supermarkets are still selling products with "dodgy discounts and misleading multibuys", according to Which?.
The consumer group analysed the prices of more than 70,000 groceries and said it found examples of "misleading multibuys that could cost you more and dodgy discounts where the offer ran for much longer than the item was at the higher price".
For example, Sainsbury's sold Carex Aloe Vera & Eucalyptus Moisturising Antibacterial Handwash (250ml) in a special offer that claimed the price "was £1.80 now 90p". It sold the item at 90p for 84 days, having previously sold it at the higher price for just seven days.
Which? also found Ocado sold a 12 pack of Beck's Bier as a "was £12.19 now £9" deal for nearly a month, when the 12 pack had only been sold at £12.19 for three days.
The research also uncovered misleading multibuys in Asda, "which didn't save customers any money or, in some cases ended up costing more", said the consumer group.
It found Asda upped the regular selling price of a four-pack of Muller Light Greek Style Yoghurts from £1.50 to £2.18 as it went on a "2 for £4" offer, resulting in additional cost of £1 for customers.
Which? said the supermarket did the same thing with the regular price of Uncle Ben's Express Basmati rice (250g) - upping it from £1 to £1.58 as it went on a "2 for £3" offer. The price fell back to £1 when the offer ended.
The consumer group pointed out that it's just one year since the government tried to stop the price tinkering by introducing new rules that eight of big supermarkets signed up to.
Not playing fair
Which? executive director, Richard Lloyd, said: "We've found dodgy discounts across the aisles, and with rising food prices hitting shoppers' budgets hard we think supermarkets are not playing fair. The stores have had long enough to sort their act out, so we're saying enough is enough, it's time to Make Special Offers Special."
Which? hopes its Make Special Offers Special campaign will put an end to misleading special offers but has threatened to "consider using its formal legal powers to ensure this poor practice is tackled" if swift action doesn't follow.