Which? found that shoppers who buy regularly from local stores instead of bigger supermarkets could be spending an extra £15.73 a week on everyday items.

A comparison revealed that 75 items including Anchor Spreadable Butter, a Hovis white bread loaf and own-brand milk bought from Tesco Express add up to £817.91 over a year.

Similarly, 69 grocery items at Sainsbury's Local, including Birdseye Potato Waffles, are costing customers an extra £477.93 annually.

Although the natural fluctuation of supermarket prices was taken into account, steep mark-ups on individual items at local stores from both retailers were found.

A Tesco own-label sweet potato was 95p on average when bought online or at a larger shop, but £1.30 on average at Tesco Express - a difference of 37%.

At Sainsbury's, the worst offender was Heinz Cream Of Tomato soup, which was £1.15 online and at a bigger shop, but £1.37 at Sainsbury's Local, a 19% mark-up.

Which? head of food policy Sue Davies said the watchdog is calling on big supermarket chains to make changes to "ensure everyone has easy access to basic, affordable food lines at a store near them".

Click to subscribe to The Ian King Business Podcast

A Sainsbury's spokeswoman said that the differences in price were down to its Local stores typically being based in city or town centre locations, where operating costs are higher.

A Tesco spokesman said: "We work hard to ensure our customers get great value at Tesco, whether they shop with us online, in a large store or in an Express store."