Which? researchers checked the prices of 20 items, including own-brand products such as eggs and cucumber, and branded goods such as Chicago Town pizzas, every day in July.

Researchers then calculated an average price for each item at each supermarket, and added up the individual item prices to generate an average trolley price for each store.

It found shoppers would have typically paid £23.59 at Aldi, just 18p less than rival discounter Lidl which charged £23.77. At £24.46, Morrisons was the cheapest of the so-called ‘big four’ supermarkets which are Morrisons, Sainsbury’s, Tesco and Asda.

The same shopping cost an average of £25.13 at Sainsbury’s, £27.06 at Tesco and £31.41 at Waitrose – the most expensive supermarket.

Groceries with some of the biggest price differences included PG Tips pyramid tea bags, which cost £1.52 more at Waitrose than at Aldi, and own-label cantaloupe melons, which cost £1.31 more.

Julie Ashfield, managing director of buying at Aldi, said: “We’re proud to be recognised for our consistent commitment to delivering our customers the best possible prices. Being named the UK’s cheapest supermarket yet again, marks our eighth win since July 2020 and we’re delighted that throughout a difficult time we’ve been able to make life a little easier for our customers.”

Which? also analysed a trolley packed with 79 items (the original 20 plus 59 more). This was made up of a wider selection of branded items, such as Amoy noodles and Lurpak butter, that aren’t always available in the discounter supermarkets. For this reason, this comparison didn’t include Aldi or Lidl.

Asda was the cheapest supermarket for the bigger trolley, with the bill averaging £151.22. It was £18.28 cheaper than the most expensive supermarket, Waitrose. Grocery prices at second-placed Sainsbury’s were only 2% higher than at Asda, with Which?’s trolley costing £154.75 – £3.53 more – at Sainsbury’s.

Which? says it can’t compare exactly the same items each month because products aren’t always available at every retailer. But Asda has been the cheapest non-discounter supermarket for a full trolley for 17 months in a row, having claimed the title every month since January 2020.