Supermarkets' “misleading” special offers will come under the spotlight at Westminster today.

Southampton Itchen MP John Denham is attempting to change the law to force major grocers to come clean about their prices.

The likes of Tesco, Sainsbury’s and Asda have been criticised by consumer groups for their tactics, with shoppers drawn in by offers that are not as good they seem.

In advance of his Supermarket Pricing Information Bill coming to the Commons today, the Labour MP has published a “dossier” showing some examples. They include: Multi-buys that do not save the shopper any money, including one chain offering a bottle of squash for £1 – and two for £2.

Prices that are hiked for a short time, then “reduced” back to the original price.

Fruit and vegetables priced in different ways, making it impossible to compare.

Larger packs that are not cheaper than buying multiple smaller packs.

Mr Denham will use his ten-minute rule Bill to call for new laws forcing the largest ten supermarkets to publish full pricing data. The information would include variations between stores, and would track price changes, allowing websites to collate the data for consumers.

Campaigners, including Which? magazine and the Consumer Action Group, believe more transparency would mean supermarkets would be forced to stick to genuine offers. However, the Bill will need considerable support in Parliament to stand a chance of becoming a law.

Mr Denham said: “My Bill will even up the relationship between supermarkets and the consumer.

“Supermarkets collect and analyse data on shopping habits to shape their pricing and promotions.

“Yet many consumers are still left shopping around from store to store to get the best deal in much the same way they did decades ago.

“Consumers will quickly be able to compare the price of their shop to get value for money, the release of this data will put an end to misleading pricing practices and deals.”

Tom Ironside, director of regulation, for the British Retail Consortium, which represents the supermarkets, said: “Consumers already have more information available to them now than ever before and the evidence is they are very sophisticated in using it to choose where to shop and what to buy.

“Prices are not secret. As part of the competitive battle between retailers, they already give customers as much price information as they can.

“We’re not convinced that a legislative requirement is needed.”