The consumer group Which? has accused Tesco of "breaking the law" with its Clubcard price promotions.
Which? has reported the supermarket chain to the Competition and Markets Authority (CMA) claiming that it does not provide detailed pricing information on its loyalty card offers.
The concerns are around the supermarket's unit pricing both in-store and online - this is the small print on shelf prices which gives the price per 100g or 100ml.
This is in place so shoppers can easily compare the value for money between different-sized products and brands.
However, on Tesco Clubcard promotional signage, Which? says this small print is not there.
Which? accuses Tesco of a lack of transparency which is making life more difficult for customers who are trying to manage higher food costs.
The consumer group claims the decision not to display this unit pricing on some offers could be a “misleading practice” under the Consumer Protection From Unfair Trading Regulations 2008.
In one example, Which? found a 700g bottle of Heinz tomato ketchup in Tesco for which the label showed the standard price to be £3.90, or 55.7p per 100g.
A Clubcard offer label also showed the same size bottle on offer at £3.50, but the unit price, which would be 50p per 100g, was not given.
At the same time a 910g bottle of the same ketchup on the shelf below was priced at £3.99, or 43.8p per 100g, for all shoppers, making it the cheapest option per 100g.
Which? argued many shoppers would wrongly assume the Clubcard option was the best deal available.
Sue Davies, the head of food policy at Which?, said: “Tesco’s unclear Clubcard pricing is at best confusing for shoppers struggling with soaring food inflation and at worst could be breaking the law.
“This is simply not good enough from the UK’s biggest supermarket. Tesco should think of its customers and act now to introduce clear unit pricing on all offers, including Clubcard promotions, so shoppers can easily find the best value items.
“We expect the regulator to look at unit pricing on the growing number of supermarket member price schemes as part of its review.
“At this time of crisis, supermarkets cannot cut corners. They have a duty to ensure pricing is clear so that customers can get the best value.”
The consumer group said it is urging Tesco to act now and introduce unit pricing on Clubcard prices as soon as possible to help shoppers find the best deals.
Which? also added that it has found issues with unit pricing across all supermarkets.
Tesco has denied the accusations from Which? and said it complies with all rules and that its approach had recently been endorsed by Trading Standards.
A Tesco spokesperson told The Mirror: "Providing great value and clear pricing is really important to us. We always take care to ensure we are compliant which is why we asked Trading Standards to review our approach on Clubcard Prices.
"They formally endorsed our labelling, confirming it meets the current legal requirements and guidelines.
"We are supportive of calls for greater clarity on the regulations in this area, in the interests of both businesses and consumers, and are actively looking at how we can make the way we display pricing even clearer for our customers.
"However, given that we are complying with all the current rules, we are disappointed that Which? has chosen to make these ill-founded claims against our Clubcard Prices scheme, which helps millions of households get great value week-in, week-out, and could save shoppers up to £351 per year."
