Asda was the cheapest supermarket for a larger trolley for 11 months of the year, with Morrisons being the cheapest in July.

Waitrose was the most expensive supermarket for 11 months of the year for a larger trolley, while Sainsbury's emerged as the most expensive for customers shopping without a loyalty card in September.

In December, the larger trolley contained 131 items, including the same 43 as the smaller basket.

The Which? analysis included special offer prices but not multi-buys or loyalty discounts, which it argued were only available to those prepared to sign up to the schemes, with not every consumer able to do so.

The watchdog said the findings demonstrated that shoppers could make considerable savings depending on where they bought their groceries.

Which? Retail editor Ele Clark said: "With food prices continuing to put immense pressure on household budgets, it's no surprise to see many people turning to discounters like Aldi and Lidl.

"Our research shows that Aldi was the cheapest supermarket of 2023, with shoppers saving almost £20 there in December compared to the most expensive store.

"Which? believes that supermarkets can do more to help shoppers during the current cost-of-living crisis. Although some have begun to make progress, we'd like to see all supermarkets stocking their smaller convenience stores with a selection of budget-range healthy foods, particularly in those areas most in need."

Julie Ashfield, managing director of buying at Aldi, said: "We're absolutely delighted to be named the cheapest supermarket of the year for a third year running.

"We are committed to providing shoppers with the best quality products at the lowest prices and it's great to have that officially recognised by a highly respected consumer champion, such as Which?"

A Waitrose spokeswoman said: "We invested a record amount of £100 million in price cuts without compromising on the quality, taste and highest animal welfare standards our customers expect from us."

Aldi has beaten rival Lidl to be named the cheapest supermarket of 2023, according to consumer group Which?

Aldi was the cheapest supermarket for 11 months of the year, with Lidl beating the rival discounter for one month in October, the consumer group said.

December's results show a basket of 43 groceries was £74.83 at Aldi, narrowly cheaper than at Lidl where it cost £76.74.

Waitrose was almost £20 more expensive than Aldi in December at £94.94 and was the most expensive supermarket every month throughout the year, Which? found.

Which? also compared the cost of a larger trolley of more than 100 items including more branded items, and did not include Aldi and Lidl as they do not always stock some of these products.