The price of own-brand and budget products in supermarkets has been rising far faster than premium and branded items during the cost of living crisis, new figures suggest.

Which? tracked tens of thousands of products across eight major supermarkets – and while budget ranges had surged by as much as 18%, the price of premium lines was up 12%.

The sharpest rise was found in the cost of Waitrose’s chocolate chip shortbread. It was priced at 82p in the year to 31 October 2021 – but one year on, this had surged to £2.25.

Meanwhile, a chilli con carne ready meal from Asda would have set you back £1.20 last year – jumping 132% to £2.79 come 2022.

While Aldi and Lidl are generally the cheapest of the big chains, Which? research suggests they are actually the worst supermarkets for overall inflation – with prices at both up by an average of about 19%.

Asda was third in these rankings on 15.2%, followed by Morrisons on 14.4%, Waitrose on 14.2%, Sainsburys on 13.7%, and Tesco on 12.6%. Ocado had the least inflation overall – but prices still rose by 10.3%.

Despite budget lines being hard hit by the cost of living crisis, Which? has urged supermarkets to work harder in ensuring they are available throughout all branches – including in smaller convenience stores.

Which? head of food policy Sue Davies said the data paints a bleak picture – but the consumer watchdog hopes it will help “millions of people find the best possible value with their weekly shop”.

She added: “We know the big supermarkets have the ability to take action and make a real difference to people struggling through the worst cost of living crisis in decades. That’s why we’re calling on them to ensure everyone has easy access to basic, affordable food lines at a store near them, can easily compare the price of products to get the best value, and that promotions are targeted at supporting people most in need”.

Aldi said the rising price of milk in its stores was due to its decision to pay more to suppliers – benefiting British dairy farmers.

Meanwhile, Lidl has disputed the data and said: “Prior to the press release being issued to media, we made Which? aware of multiple discrepancies within the data concerning Lidl prices. This included over 100 separate pricing inaccuracies, which were clearly highlighted to Which? Despite this, they have chosen to mislead consumers by reporting data that they know to be incorrect.”

Asda said it was focused on keeping the prices of branded and own-label products in check, with Morrisons vowing to remain competitive during an “unprecedented period of inflation”.

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Waitrose warned “no retailer is immune to inflation”, with rising production costs and the war in Ukraine just some of the factors that have driven prices up. Meanwhile, Sainsburys said it is investing £500m to reduce the price of items that its customers buy the most.

A Tesco spokesperson said it has locked the price of more than 1,000 household staples until 2023 and is “keeping a laser focus on the cost of the weekly shop”.

Ocado declined to comment.