Eight leading supermarkets have signed up to a set of principles to ensure that shoppers are not misled by claims on special offers.
The Office of Fair Trading launched a inquiry in February to examine concerns that people were confused by the way prices are displayed in the £163.2bn grocery industry.
Tesco, Morrisons, Sainsbury's, Marks & Spencer, Co-op, Lidl, Aldi and Waitrose agreed to adopt the OFT's voluntary code – which is not legally binding – but Asda, the second-biggest operator, refused to sign up.
The new code covers claims such as "was £3, now £2" and "half-price". It states that prices should be presented as discounts for the same or less time as a product was initially sold at the higher price. But retail experts doubt whether the guidelines will make much difference in an industry that has always blitzed consumers with promotions.
Bryan Roberts, of Kantar Retail, said: "Even with a new voluntary code and greater regulation, it will still be confusing to ascertain what value for money looks like and what is a bargain and what is not. Most shoppers will still need a calculator to work their way around a store."