Eight supermarkets have agreed to make sure that special offers and price promotions on food and drink are fair.
It follows an investigation by the Office of Fair Trading (OFT) into the way prices are displayed and promoted in stores.
Concerns have been raised about prices being temporarily inflated to make later discounts look more attractive.
Major UK supermarkets have now agreed to follow a set of principles drawn up by the OFT.
Tesco, Sainsbury's, Morrisons, Waitrose, Marks and Spencer, Aldi, the Co-op and Lidl have all signed up.
Asda says it is reviewing the proposed code.
The principles cover claims such as "Was £3, Now £2" or "Half Price", which must now only be offered for the same or less time than the product was initially sold at the higher price.
The supermarkets have also agreed not to artificially inflate prices to make a later "discount" look more attractive.
Pre-printed claims on packs, such as "Bigger Pack, Better Value", must be true - and there should not be a cheaper way of buying the same volume of the product elsewhere in the same store, even if there is a promotion on smaller packs of the same item.
The OFT did not find that the supermarkets had breached the law or were engaging in misleading promotional practices but did find "what appeared to be inconsistency in the way the law was being interpreted and applied", it said.
OFT chief executive Clive Maxwell said: "Household budgets across the country are under pressure and shoppers should be able to trust that special offers and promotions really are bargains.
"Prices and promotions need to be fair and meaningful so shoppers can make the right decisions."