A simple shopping mistake at Tesco and Sainsbury's is costing consumers £818 each a year. Opting to shop at a Tesco Express or Sainsbury's local rather than larger supermarket store has been hitting consumers hard in the pocket, according to Which? analysis. Consumers buying the same 75 items at Tesco Express instead of a larger Tesco spend an extra £15.73 a week. And Sainsbury's Local shoppers are being hit with 19 per cent hikes on some items. Sue Davies, head of food policy at Which?, said: "The big supermarkets have the ability to take action and make a real difference to people struggling through the worst cost-of-living crisis in decades. That's why we're calling on them to ensure everyone has easy access to basic, affordable food lines at a store near them, can easily compare the price of products to get the best value and that promotions are targeted at supporting people most in need."