Convenience shops like Tesco Express and Sainsbury's Local are likely to cost consumers hundreds of pounds more a year, a study suggests.
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".
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."
(c) Sky News 2023: Shopping at convenience stores instead of bigger supermarkets could cost you an extra
Five Memorable Oasis Performances To Get Us In The Mood
Five Amazing Places To Visit In September
Five Incredible Waterfalls You Need to Visit
Five Amazing Barbecue Dishes to Try from Around the World
Five Strangest Theme Park Rides Around the World