Which? carried out a survey, and nearly eight out of ten people said that their weekly shopping bill has increased in the past year, and nine out of ten said they shop around for the best deal.
The law currently states that retailers must provide both a selling price (the price that the product actually costs) and a unit price (the price it costs by volume or weight) for food.
88 percent of those surveyed use unit pricing to choose which product to buy, but this can be harder than people think. For example, £1.89 jar of light mayonnaise may be priced at £4.73 per kg, and a £1.00 jar of a different brand of light mayonnaise could cost 21p per 100ml. As the unit prices are in different measurements, consumers would be unable to make a comparison, short of turning up with a calculator and a units converter.
Richard Lloyd, executive director of Which?, said: "With household budgets squeezed and rising food costs among the top worries for consumers, it's all the more important that stores make it as easy as possible for people to spot the best value products.
"All food and drink should be clearly and consistently priced by weight or volume across all stores, including products which are on special offer."
If you want to support the campaign, you can sign the pledge at which.co.uk/unitpricing. People can also tweet pictures of confusing unit pricing to @whichaction using the #priceitright.