The consumer champion tracked hundreds of thousands of grocery prices across the UK’s eight major supermarkets throughout 2021 to find out how much each shop was charging for everyday items such as bread, milk and eggs.

As well as analysing the data to determine the cheapest supermarket, Which?’s analysis also shows that prices have been creeping up, with supermarkets charging up to 9% more in December than they did last January.

Overall Aldi was the cheapest supermarket for six of the 12 months while Lidl was the cheapest for five including December 2021. For one month, January 2021, Aldi and Lidl were tied with a basket of 19 items coming in at £18.45 at both budget supermarkets.

Lidl, winner of the cheapest supermarket accolade for 2020, was the cheapest supermarket in December, at £23.29 for a basket of 22 groceries, just beating Aldi, where the basket was £23.64.

Meanwhile, Waitrose was more than £9 pricier than Lidl, at £32.85 – that’s 41% more. In fact, Waitrose was consistently the most expensive across the 12 months. A basket of everyday items cost from £6 to over £10 more at Waitrose per month than the cheapest supermarket.

Alongside the price comparison of a basket of groceries at all eight supermarkets, Which? also compared a larger trolley packed with a greater selection of items such as Cathedral City cheddar and Kenco coffee, that are not always available at the discounters (Aldi and Lidl) – meaning they cannot be included in this bigger comparison.

Asda was the cheapest of the traditional supermarkets and has been for every month for the past two years. Waitrose was also the most expensive each month except one in 2021 for this wider analysis.

The analysis also revealed that prices had risen significantly over the course of the past 12 months. Which? found that prices for the basket of everyday items had risen the most at Waitrose – by a massive 9%, compared to the average 3% rise across all eight supermarkets.

Even prices at Aldi and Lidl rose between January and December 2021 – in both cases at more than the average rate.

Ele Clark, Which? retail editor, said: “No one wants to overpay for basic groceries, especially when a cost of living crunch is putting extra pressure on household budgets.

“Our findings show that while prices are going up, some supermarkets are passing their rising costs onto shoppers more than others. As well as choosing a supermarket that is cheap overall, other ways to save include swapping from branded to own-brand products, sticking to a shopping list and resisting the temptation to pick up special offers you don’t need.”

Julie Ashfield, managing director of buying at Aldi, said: “We’re absolutely thrilled to be named the Cheapest Supermarket of the Year. We are committed to ensuring that our customers have access to the highest quality products at the best possible prices. This is a commitment to our customers that is set in stone and we’re proud that it has been recognised by Which?, supporting shoppers at a time when the cost of living is rising.”

Aldi opened its first checkout-free store earlier this week.