There is an almost £9 difference in cost for an identical basket of groceries at the most expensive and cheapest supermarkets, a study has found.
Which? Money compared a basket of 10 branded essential items at six leading grocers, finding Waitrose charged the most at £32.85 and Asda charged the least at £24.12 - a saving of £8.73 or 36%.
Asda's basket also came in at £6.32 less than the next cheapest basket, costing £30.44 from Tesco.
The biggest price variation for an individual product was for a 750ml jar of Hellman's mayonnaise, which cost £2 in Asda and £4.42 in Morrisons - a difference of 221%.
The research also found 14 different packets containing identical caplets of the painkiller Ibuprofen - which is subject to strict rules governing the active ingredients - with prices ranging from 8p a pill to 20p.
Which? Money editor Harry Rose said: "With food prices rising well beyond the general rate of inflation, we're all looking for ways we can cut down on the cost of our weekly shop."
A Waitrose spokeswoman said: "Our prices for these brands are very competitive - this price comparison is distorted by the timings of others' promotions."
Prices were compared on March 8 using the comparison site Mysupermarket.co.uk.