Aldi was the cheapest supermarket in October, according to the latest monthly analysis from Which?.
We compared prices for a basket of 23 items throughout last month and found that, on average, shoppers would have paid £24.24 at Aldi. It beat rival discounter Lidl by just 73p.
Meanwhile, the most expensive supermarket was more than £9 pricier than Aldi for an equivalent basket of groceries. Here, we reveal the month's cheapest and most expensive supermarkets.
Every day throughout October we checked the prices of 23 items, including own-brand products such as apples and eggs as well as branded goods such as Hovis wholemeal bread, to see how the UK's biggest supermarket chains compared.
Here's how much our basket cost on average:
Aldi () came out cheapest overall, at just £24.24. Asda () was the cheapest 'big four' supermarket, with our basket costing £25.94. At the other end of the scale, Waitrose was more than £9 pricier than Aldi, at £33.81 - that's 39% more.
Groceries with some of the biggest price differences included own-label seedless grapes and own-label melon, both of which had a difference of £1.11 between Aldi and Waitrose, and PG Tips tea bags, which had a difference of 94p.
Of course, price is just one factor when you're deciding which supermarket to shop at. We also survey shoppers on their experiences of supermarkets' product quality, customer service, store experience, online deliveries and a range of other factors to reveal the each year.
We also compared a trolley packed with 77 items (the original 23 plus 54 more). This included a greater selection of branded items, such as Cathedral City cheddar and Colgate toothpaste, that aren't always available in the discounter supermarkets - so for our bigger trolley we haven't been able to include Aldi or Lidl.
Asda, at £143.21, was easily the cheapest of the traditional supermarkets. It was £21.70 cheaper than the most expensive supermarket, Waitrose.
Grocery prices at Sainsbury's weren't much more than at Asda this month: our trolley came to £145.56.
We can't compare exactly the same items each month because products aren't always available at every retailer, but Asda has been the cheapest mainstream (non-discounter) supermarket for 22 months now, having claimed the title every month since January 2020.
We check the prices of hundreds of grocery items at eight major supermarkets every day throughout the year, using an independent price comparison website.
For our 'cheapest supermarket of the month' analysis, we work out the average price for each item at each supermarket across the month, and add the averages up to get an average trolley price for each store.
We include special offer prices but not multibuys, to keep it as fair as possible.
Our shopping list combines branded items such as Kenco coffee, Oxo stock cubes and Twinings English breakfast tea with own-label products, including onions and milk. Of course own-brand items aren't exactly the same at different supermarkets, but we've used experts to ensure that the products are as comparable as possible based on a range of factors, including weight, quality and other industry data.
The past month has seen increased concerns about rising grocery prices.
Like-for-like grocery prices rose by 1.7% in the four weeks to October 3, compared with the same time last year, according to market analyst Kantar.
Spokesperson Fraser McKevitt said: 'In real-world terms, the average household had to spend an extra £5.94 on groceries last month than they did at the same time last year. The typical household spends £4,726 per year in the supermarkets, so any future price rises will quickly add up.'
Meanwhile several big food and drink companies said they were preparing to increase prices, including P&G, Kraft Heinz, Mondelez, General Mills and Colgate-Palmolive due to supply chain pressures.