The consumer watchdog found that on average, shoppers would have paid just 43p extra at Aldi compared to Lidl for a basket of 23 items.
It also found that the most expensive supermarket, Waitrose, was more than £9 pricier than Lidl for an equivalent basket of groceries.
Which? compared baskets of essential items that included each supermarket’s own-brand products, such as apples and eggs, as well as branded items such as Hovis wholemeal bread.
It revealed that the basket would cost £24.11 at Lidl, compared to £25.54 at Aldi. Asda was the cheapest of the “big four” supermarkets, costing £25.22 for the equivalent basket.
The basket of Waitrose items cost 38 per cent more than Lidl, at £33.20.
Items with some of the biggest differences in price included own-brand melon, which cost £1.31 more at Waitrose compared to Lidl, and PG Tips tea bags, which had a difference of £1.25.
Which? also compared the price differences of 82 items to determine the cost of a bigger shopping trip. However, this trolley contained a greater selection of branded items that aren’t always available at discount supermarkets, and so the analysis did not include Aldi or Lidl.
Asda came out on top for the cheapest of the traditional supermarkets for a big grocery shop, at £149.20, and has remained the cheapest mainstream supermarket since January 2020. In contrast, Waitrose cost £21.22 more for an equivalent trolley.
Prices in Sainsbury’s, Morrisons, Ocado and Tesco were similar, with just £2.63 between them.
Which? checks and compares the prices of grocery items at every major supermarket throughout the year, releasing their cheapest supermarket of the month analysis regularly.
The latest analysis comes as the UK grapples with a nationwide shortage of drivers. Some supermarkets and hauliers have warned that the shortage may lead to higher prices on groceries in the coming months.
Tesco chairman John Allan and Iceland managing director Richard Walker were among retail bosses to warn that their supermarkets have been impacted by the shortage of HGV drivers last month.