In total, the basket of 42 groceries would have averaged around £11 more at Sainsbury’s Local and £10 more at Tesco Express for loyalty scheme members, compared to their price

Researchers also compared prices for customers with loyalty memberships such as Sainsbury's Nectar, Tesco Clubcard and Morrisons More. Morrisons customers face the biggest price difference as research found that consumers pay 21 percent more for the same groceries at a Morrisons Daily than at its supermarket.

An investigation has found that Tesco, Morrisons and Sainsbury's all charged more for a basket of shopping in their smaller local convenience stores.

Thousands are wasting up to £832 a year by shopping at the wrong supermarket. Which? compared the cost of 42 popular grocery items at convenience chains Morrisons Daily, Sainsbury's Local and Tesco Express against prices at their full-sized supermarkets.

A survey found that those who do their grocery shopping at convenience stores could be paying more than those who stick to a big shop at a larger supermarket