Aldi, Co-Op, Lidl, Marks and Spencer, Morrisons, Sainsbury's, Tesco and Waitrose have agreed to make promotions on their food and drink fairer.

But there is one notable omission - Asda is the only major supermarket chain not to adopt the principles from the Office of Fair Trading.

In a statement Asda claimed it was considering signing up, but as it aims to keep prices for customers "as low as possible for them week in week out", a code covering special offer price promotions was not relevant.

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The principles clarify the OFT's view on how promotional claims should be used, so that consumers can rely on them being fair and meaningful regarding the value of the product or the existence of a discount.

In particular, the OFT has asked the supermarkets to ensure that when using internal reference pricing, such as 'Was £3, Now £2' or 'Half Price', the new prices should be presented as discounts for the same or less time than the product was initially sold at the higher price, and prices should not be artificially inflated to make a later 'discount' look more attractive.

Similarly, claims on packs that they are better value because you're buying a bigger quantity must be provably true.

The OFT said in a statement: "Where such claims are made, there should not be a cheaper way of buying the same volume of the product elsewhere in the same store. This applies even when there is a promotion on smaller packs of the same item."

Importantly, the OFT's investigation found that none of the supermarkets were breaching the law or engaging in misleading promotional practices, but there was an inconsistency in the way the law was being interpreted and applied. It developed these principles to establish a more consistent approach across the sector.

Clive Maxwell, OFT's chief executive, said it was important for shoppers to be able to trust that special offers and promotions really were bargains.

"Prices and promotions need to be fair and meaningful so shoppers can make the right decisions. Nowhere is this more important than during regular shopping for groceries, which accounts for 44% of household spending," he said in a statement.

"Our principles, taken together with previous guidance, provide supermarkets with a clear benchmark for how they should be operating so that their food and drink promotions reflect the spirit as well as the letter of the law. We are pleased that supermarkets have engaged constructively throughout our investigation and we will keep a watching brief on promotional practices in this sector."

UPDATE:
Asda has been in touch with Huff Post UK and told us: "At Asda we are committed to giving our customers clear and accurate pricing information that fully complies with the law. As an everyday low price retailer, we will always focus on offering our customers the lowest prices week in, week out. We welcome the work carried out by the OFT in this area and are taking the time to consider its recommendations in detail."

Asda in fact agrees with three of the four principles put forward by the OFT, but struggles with the 1:1 principle, which says goods must be advertised at a discount price for no longer a period than it had been advertised at originally.

An Asda spokesman told Huff Post UK: "We were part of the discussions around the principles and are very much engaged with the OFT's work in this area, but while we are in favour of three of the approaches, there is one, regarding 1 to 1 promotions that we’d like a bit of time to consider.

"Essentially, it says that you can only run a promotion for as long as you have established the price for. Whilst this is meant to discourage high/low pricing promotions – our concern is that it could actually encourage it. If so, we're not sure that best helps customers in these challenging times, so we are taking the time to consider its proposals in detail."