Iceland and Marks & Spencer have resisted moves to make their pricing more transparent after Aldi, The Co-operative and Waitrose today joined the Price it Right campaign.

The Which? Price it Right campaign, which launched in September, calls for simpler, clearer and consistent pricing for food products in supermarkets.

The consumer group wants to “end hard-to-read and inconsistent pricing that confuses consumers and makes it difficult for shoppers to identify value for money”. It calls for the unit price of food items to be on in-store promotions.

Morrisons joined the voluntary campaign when it launched but the remainder of the big four grocers are yet to sign up.

Of the six other major grocers, Which? said Sainsbury’s has been working on transparent pricing over the past 18 months and Lidl and Tesco are making active steps towards simpler pricing.

Asda has said it will look at improving its labelling but Iceland and Marks & Spencer have still yet to take any action, the consumer body said.

New research from Which? shows 78% of people shop around for the best price for their groceries and 43% are now comparing prices when shopping in store, more than they were a year ago.

Business secretary Vince Cable said: “I welcome the positive steps taken by supermarkets to help consumers get the best value and a fairer deal. Clearer prices and promotions will be a big help for consumers who want to be more savvy and work out how to save more on their weekly shop. In addition, more transparency will boost competition and help support a stronger economy.

“Consumer minister Jo Swinson has been working closely with the supermarkets and with Which? to explore what more can be done to make prices simpler. This is an encouraging first step and I look forward to other supermarkets following suit.”

Co-operative Food chief operating officer Sean Toal said: “Later this year, we plan to roll out enhanced shelf-edge labelling, displaying larger, clearer and more visible unit prices for products, in all of our 2,800 food stores nationwide.

“This move will enable shoppers to more easily compare the prices of similar products, so they can make more informed choices. Unit prices will also be more clearly displayed for products that are on promotion, allowing shoppers to better understand the savings that can be made.”