The news comes as Which? also reveals that grocery prices have risen by up to 9% over the past 12 months, in real terms.
The research – which sees the consumer champion comparing similar items from across all the major UK supermarkets – revealed that Aldi was the cheapest supermarket for six of the last 12 months while Lidl was the cheapest for five, including December 2021.
The pair also tied for one month – January 2021 – when both ‘baskets’ came in at £18.45.
Which? also tracked hundreds of thousands of grocery prices across the UK’s eight big supermarkets, finding that some items cost as much as 9% more in December than they did in January.
Waitrose prices increased the most (by 9.2%), while Sainsbury’s mostly held firm (at 0.59%). On average, prices rose by an average of 3.4%.
Waitrose was also consistently the most expensive supermarket across the 12 month period, with a basket of everyday items costing between £6 and £10 more per month than the cheapest alternative.
The typical basket shopping list used by Which? to compare the supermarkets featured a mix of branded items (such as Kenco coffee, Oxo stock cubes and PG Tips tea bags) alongside own-label products such as onions and milk.
Which? also compared a larger trolley with a greater selection of items which could not always be found at either Aldi or Lidl. For this, Asda was the cheapest of the traditional supermarkets, with Waitrose again the most expensive.
“No one wants to overpay for basic groceries, especially when a cost-of-living crunch is putting extra pressure on household budgets,” commented Which? retail editor Ele Clark.
“Our findings show that while prices are going up, some supermarkets are passing their rising costs on to shoppers more than others.”