The consumer group says Tesco could be breaking the law by not being clear enough about its unit pricing.
Consumer group Which? has reported Tesco to the UK 's competition watchdog over its Clubcard prices.
The Tesco Clubcard is a loyalty scheme that offers members discounted prices on products.
Which? says the supermarket does not explain the unit price of deals clearly enough to shoppers, making it hard for them to determine the cheapest product.
In response, Tesco said it complied with all current rules and said Which?'s claims were "ill-founded".
Heinz tomato ketchup was one of the items that Which? highlighted.
The consumer group found a 700g bottle in Tesco for which the label showed the standard price to be £3.90, or 55.7p per 100g.
A prominent Clubcard label showed the same size bottle on offer at £3.50, but the unit price, which would be 50p per 100g, was not given.
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.
The consumer group said Tesco's decision not to display unit pricing on its Clubcard offers could be breaking the law.
According to competition rules, unit prices could be seen as "material information" which most people would need in order to make an informed decision about how to get the best value from what they are buying.
A spokesperson for Tesco said the company had sought advice and approval from its local trading standards office in Hertfordshire where its headquarters are based.
"Providing great value and clear pricing is really important to us, we are supportive of calls for greater clarity on the regulations in this area," it said.
"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."
The Competition Markets Authority (CMA) is currently investigating whether supermarkets are making excess profits through inflated prices.
In May, the boss of Sainsbury's, Simon Roberts, told the BBC that the supermarkets were not profiteering from high inflation.