Supermarkets have come under fire from the competition regulator for fake deals and bogus bargains.
They have also been warned that they could face legal action for misleading customers with dodgy multi-buys, shrinking products and confusing discounts.
It comes after a three-month investigation by the Competition Markets Authority (CMA) found that some promotions and confusing unit pricing had the potential to mislead customers and could lead to a breach of consumer law.
Of every £5 consumers spend in a large grocer, £2 is on something that's part of a deal.
The problem is these so-called offers are effectively fake.
Following a complaint by the consumer association Which?, the UK's competition watchdog found evidence of misleading pricing and promotional practices that could be against the law.
While these problems did not occur in large numbers across the sector, there were still areas of poor practice that could confuse or mislead shoppers, the CMA said.
Which? executive director Richard Lloyd said it was time for the big grocers to clean up their acts.
"The CMA's report confirms what our research has repeatedly highlighted: there are hundreds of misleading offers on the shelves every day that do not comply with the rules," he said.
"This puts supermarkets on notice to clean up their pricing practices or face legal action."
The CMA found that promotions such as 'was/now' offers, where a product is on sale at a discounted price for longer than the higher price applied, were misleading.
It also uncovered evidence that certain retailers increased the price of products prior to the launch of a volume promotion. Furthermore, it said more could be done to reduce the complexity in unit pricing, as this was making it difficult for customers to compare prices.
The CMA said it was taking a series of measures to help make sure grocers complied with rules, improve clarity for shoppers and simplify regulations.
Its report said: "If this further work reveals clear evidence of breaches of consumer law, this could lead to enforcement action."
Nisha Arora, CMA senior director for the consumer sector, said urgent action was required.
"We have found that, whilst shoppers enjoy a wide range of choices, with an estimated 40% of grocery spending being on items on promotion, there are still areas of poor practice that could confuse or mislead shoppers," she said.
"So we are recommending further action to improve compliance and ensure that shoppers have clear, accurate information"'
The CMA has also called for new guidelines to be issued to supermarkets and has published its own 'at a glance' guidance to help consumers.
There are two main ways supermarkets use "deals" to try and trick consumers into spending more, or thinking something is better value.
1. Was/Now pricing promotions
Frequently the discount price is advertised for longer than the higher price. For example: Pepsi Max (2 litres) advertised at £1.98 for 28 days but then went on a £1 "was £1.98" offer for 63 days.
2. Unit pricing
Unless you know what you're paying per unit you can't compare different products - but Which? found cases where some sauces in supermarkets priced per 100 ml and on others per 100g, making it impossible to make a price comparison.
Sainsbury's sold a pack of four baking potatoes for 75p, or 19p each. If you bought five of them loose, however, they'd be 36p each
Asda increased the regular price of Uncle Ben's Basmati rice from £1 to £1.58 at the same time as offering 'Two for £3'.
Tesco was fined £300,000 in 2013 over a half-price strawberry promotion. It charged an 'original' price of £3.99 for a short period before a half-price promotion at £1.99 which ran for weeks