With UK food inflation close to record highs, Britons are shopping around and comparing prices at different supermarket groups.
Which? said on Friday that Tesco's policy not to include unit pricing, the price per 100g or 100ml, for example, on its Clubcard loyalty scheme offers could in some cases be a misleading practice under consumer protection regulations.
"Unit pricing helps shoppers compare the prices of different products and make informed decisions about what to buy," said Which?.
"This is particularly crucial during the current cost of living crisis as grocery inflation has hit historical highs."
Tesco, which has a more than 27% share of Britain's grocery market, said it complied with all the rules and that its approach had recently been endorsed by Trading Standards, a government service that ensures consumers are protected from unfair trading.
A Tesco spokesperson said the company was disappointed with the "ill-founded" claims against the Clubcard Prices scheme.
"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," the spokesperson said.
They said that Tesco was, however, actively looking at how it could make the way it displays prices even clearer.
Britain's Competition and Markets Authority (CMA) launched a review of supermarkets' unit pricing in January, and last month said it would step up its examination of grocery prices. It said it had so far not seen evidence pointing to specific concerns and will update on its work in late July.
According to the most recent official data, food and drink inflation was 19.1% in April, just below a 46-year high.
Grocery inflation eased slightly to 17.2% in May, industry data showed.
Shares in Tesco, which is scheduled to update on first quarter trading on June 16, were down 0.4% in morning trading, paring 2023 gains to 16.4%.