The research suggests prices are raised just days before the deals are introduced
Both supermarkets denied they had been using "dodgy" pricing and denied any wrongdoings.
A Tesco spokesperson said: 'We know that having low prices on the products we sell is really important to our customers right now, which is why we have more than 8,000 weekly deals on Clubcard Prices, offering customers potential savings of up to £351 a year – all while collecting Clubcard points that can be put towards groceries and fuel, or doubled in value with our Reward Partners.
"All our Clubcard Price promotions follow strict rules, including considering how they compare against prices in the market, to ensure they represent genuine value and savings for our Clubcard members. These rules have been endorsed by our Trading Standards Primary Authority.
"As Which? reported recently, Tesco was the cheapest of all the major supermarkets when a Clubcard was used – and was extremely competitive when compared with the limited range discounters."
A Sainsbury's spokesperson added: "Nectar Prices offer our customers the opportunity to make genuine savings across 5,000 products.
"Which? fails to recognise that base prices have been increasing throughout the year due to inflation. Our promotional rules around Nectar Prices are informed by the guidance from Trading Standards.
"The Nescafe Gold example demonstrates Which?'s flawed methodology as the claim that the 'regular' price was £6 is untrue.
"The base price of this item has been £8.10 since December 2022 and £6 was a promotional price throughout this year, including on Nectar Prices when it launched in April."