Lidl has taken the top spot in Which?'s latest monthly supermarket price comparison, with a basket of groceries costing just £22.66.
We compared prices for a basket of 20 products throughout May. Lidl narrowly beat rival discounter Aldi by just 40p.
Here, we reveal which supermarket charged shoppers £9.22 more than Lidl for an equivalent basket, plus the 'big four' supermarket that was cheapest for a full trolley of 82 items.
Every day throughout May we checked the prices of 20 items, including own-label products such as tomatoes and chicken drumsticks and branded goods such as Nescafe coffee, to see how UK supermarkets compared. Here's how much our basket cost on average:
Groceries with some of the biggest price differences included PG Tips tea bags, which had a difference of £1.52 between Lidl and Waitrose, own-brand raspberries with a £1.20 difference, and large free-range eggs, which had a £1.13 difference.
However, price is just one element to consider when you're deciding which supermarket to spend your money at. We also survey shoppers about quality, customer service, store experience, online deliveries and a range of other factors to reveal the each year.
Which? also evaluated a bigger shopping trolley packed with 82 items (the original 20, plus 62 more).
This trolley included a greater selection of branded food and drink that aren't always stocked in the discounter supermarkets, which is why we haven't been able to include Aldi or Lidl in this chart.
Asda was also a whopping £22.49 cheaper than the most expensive supermarket, Waitrose, which charged 13% more at £181.60.
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 'big four' supermarket for more than a year now, having claimed the title every month since January 2020.
Our trolley of food items changes each month, as products must be available at every retailer for us to include them in the analysis.
Our shopping lists combines branded groceries, such as Chicago Town pizzas and Hovis bread, with own-label products such as cucumber and milk. Although own-brand items aren't exactly the same at different supermarkets, 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.
Using an independent price comparison website, we also calculate the average price (including special offers but not multibuys) for each product throughout the month. We then add those individual averages together to give an overall price for the trolley at each shop.
People have spent an additional £3.8bn in supermarkets over the past three months compared with the same period in 2019, according to data from market research company Kantar Worldpanel.
Kantar said the surge has come thanks to the continued rapid roll-out of the Covid-19 vaccination and the consequential increase in how safe people feel when shopping.
People also made 58m more visits to the supermarket this May than they did in May 2020.
Grocery shopping spend, however, fell 0.4% during the 12 weeks to 16 May 2021.
Fraser McKevitt, head of retail and consumer insight at Kantar, said the year-on-year decline is a result of the weeks of in the first few months of 2020. 'The data reflects a tough comparison with exceptionally high sales during the first three months of the pandemic last year,' he said.
The decline is also partly down to the fact that restaurants, pubs and cafes are opening back up again, with people eating out as well as at home.
Other grocery shopping habits developed during the pandemic are changing too. The proportion of supermarket sales made online is much higher than 2019 levels, but fell back from 13.9% in April to 13.4% in May.
Convenience stores are seeing a drop in popularity as well, as they now account for 12.5% of sales, down from 14.9% in May 2020.