The competition watchdog is set to announce new enforcement measures in response to a super-complaint by Which? that accused supermarkets of ripping off consumers with misleading and confusing prices.
Which? lodged the complaint with the Competition and Markets Authority (CMA) in April last year, claiming retailers were creating the illusion of savings through the use of multi-buys, shrinking products and baffling sales offers.
Following a three-month probe, the CMA found that supermarkets were misleading customers with confusing pricing promotions that could be against the law.
It found "areas of poor practice that could confuse or mislead shoppers" and said it was taking measures to help make sure grocers complied with the rules.
It warned of enforcement action where there was evidence of breaches of consumer law.
The CMA said it will announce "follow-up work to the Which? super-complaint about misleading and opaque pricing practices" on Wednesday.
Which? said 40% of groceries were 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 were misleading.
It added that it was virtually impossible for people to know if they were getting a fair deal, particularly when prices varied frequently or when consumers were in a rush and might 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.
Promotions have become more widely used in the industry as major supermarkets have cut prices to fight losses in market share to discounters such as Aldi and Lidl.