Shoppers are missing out on the best value-for-money products because of the way they are priced.
Consumer watchdog Which? is calling on supermarkets to clearly show “unit” prices to make it easier to select the cheapest version of the same item.
For example, it found up to 17 different sizes of Coca-Cola on shelves with prices varying between 11p and 50p per 100ml.
It means shoppers choosing four 250ml glass bottles for £5 pay 346% more than those putting a 1.5 litre for £1.68 into their baskets.
Which? also found that the price per 100ml of own-label semi skimmed milk varied between 6p and 13p - that works out 133% more for a shopper choosing a 500ml bottle for 65p than one picking a 2.27 litre bottle for £1.27.
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Its survey of 2,000 Brits found almost three quarters struggled to figure out the cheapest size in a range of products, highlighting the need for clarity on the shelves.
Unit pricing simply shows the cost per unit of weight and the consumer champion said it makes it much easier for shoppers to see at a glance if a larger size is better or worse value than two smaller ones.
As the cost of living continues to spiral out of control, the watchdog said it is vital that shoppers can quickly work out which pack sizes offer the best deal.
Its probe also found confusion around the language used by stores with the term “each” referring to both a single product like a pear or a pack of four pears.
Which? examined the cost of 10 everyday items at four major supermarkets and nine supermarket websites to see how easy it was to work out the best value size of the same product based on unit prices.
Sue Davies, Which? head of food policy, said: “At a time when food prices are a huge concern, unit pricing can be a useful tool for shoppers to compare and choose the cheapest groceries but unclear supermarket pricing means the vast majority of people are left struggling to find the best deal.
“Small savings can add up and make a big difference but unless supermarkets make unit pricing much more prominent, legible and consistent - as well as displaying it on their promotional offers - people will continue to risk missing out on getting the best value.”
A Lidl spokesperson said: “We always endeavour to ensure that pricing information is as clear as possible for our customers so that they can make informed purchasing decisions.”
A Waitrose spokesperson said: “We regularly review all our products to ensure our unit pricing is clear and consistent, so that customers can compare prices and save money.
“Our Partners are always on hand to assist customers with any pricing queries.”
A Morrisons spokesperson said: ”We show unit pricing on our milk point of sale.”
A spokesperson from Tesco told The Mirror that the supermarkets sells sell a wide range of products at different price points.
Which? also approached Amazon Fresh, Asda, Iceland, M&S, Ocado and Sainsbury's.
