Lidl was the cheapest supermarket in March, according to the latest monthly analysis from Which?.
We compared prices for a basket of 21 items every day last month and found that, on average, shoppers would have paid £26.83 at Lidl. It beat rival discounter Aldi by just 31p. Meanwhile, the most expensive supermarket was £9.21 pricier than Lidl for an equivalent basket of groceries. It comes as grocery prices are rising at their fastest rate in a decade.
Here, we reveal the month's cheapest and most expensive supermarkets.
Every day throughout March we checked the prices of 21 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:
Lidl () came out cheapest overall, at just £26.83. Asda () was the cheapest 'big four' supermarket, with our basket costing £28.63. At the other end of the scale, Waitrose was £9.21 pricier than Lidl, at £36.04 - that's 34% more.
Groceries with some of the biggest price differences included own label salmon fillets, which had a difference of £1.52 between Lidl and Waitrose, and own-label seedless grapes, which had a difference of £1.41.
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 65 items (the original 21 plus 44 more). This included a greater selection of branded items, including Twinings tea bags, 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 £128.60, was the cheapest of the traditional supermarkets. It was £20.16 cheaper than the most expensive supermarket, Waitrose.
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 27 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.
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 and quality.
Grocery prices are rising at their fastest rate for a decade. Grocery price inflation was 5.2% in March, according to the latest figures from analysts Kantar - the highest level since April 2012. Prices are rising fastest in markets such as savoury snacks, dog food and cat food, while falling in fresh bacon.
Kantar added consumers were increasingly turning to own-label products, which are usually cheaper, rather than branded. Supermarkets are also adapting their pricing strategies and moving away from selling groceries at 'round pound' prices such as £1, £2 and £3.
On top of this, supply chain pressures and the potential impact of the conflict in Ukraine (which has seen surging prices for oil, fertiliser, wheat and corn) are set to continue pushing up prices. There are particular issues on UK shelves with shortages of sunflower oil and free-range eggs.