Shopping at a local convenience store can cost you up to a fifth more than buying the same items at larger branches of the supermarket chains who own them, according to a new study – with the difference even greater for loyalty scheme members.

Which? compared the cost of 42 popular grocery items – from cheese and pasta to blueberries and ice-cream – on three occasions in June and July at the largest supermarket-branded convenience chains – Morrisons Daily, Sainsbury’s Local and Tesco Express – against prices at their full-sized supermarkets.

The biggest price difference for the items compared by the consumer group – branded and own-label – was at Morrisons. It found shoppers would pay 21% more on average for the same groceries at a Morrisons Daily than at one of its large supermarkets.

A 400g tin of own-label chickpeas was more than double the price at the convenience store – £1 in comparison with 49p at the supermarket.

Members of Morrisons’ loyalty scheme More could pay 22% extra at its smaller outlets as the card is not currently accepted there.

Morrisons said it was “actively working” on introducing the More card to Morrisons Daily stores “in the coming months”.

A Morrisons spokesperson said: “We’re always working hard to keep prices down and competitive for our customers while maintaining high standards and availability in all our stores. Last year, we became the first supermarket to introduce our budget ‘Savers’ range into Morrisons Daily stores nationwide.”

Ele Clark, the retail editor at Which?, said: “Unfortunately, many people are without easy access to transport or online deliveries which leaves them reliant on smaller nearby stores.

“Convenience stores may often be easier to travel to and handy for shoppers who need to stock up on a few essentials, but people who have to use them regularly will be spending significantly more over the course of a year than those with access to larger supermarkets.”

Which? found those who shop regularly in smaller outlets face a big mark-up on essentials such as milk, bread and butter.

All three supermarket convenience stores charged 8% more for two pints of own-label, semi-skimmed milk – with or without a loyalty card – according to the study.

Both Tesco and Sainsbury’s said their smaller stores tended to be in city or town centre locations where rents, rates and other operating costs are higher relative to their size. Both said that the difference in prices of some products reflected these extra costs.

Tesco and Sainsbury’s were found to have fewer price differences on the key items, with the average price difference 10% at Tesco and 5% at Sainsbury’s. However, the price difference was far greater for those signed up to the two chains’ loyalty schemes.

Members of Sainsbury’s Nectar scheme could pay an average of 14% more at its Sainsbury’s Local convenience stores, while Tesco Clubcard holders could find items 11% pricier in one of its smaller Tesco Express outlets than in its large supermarkets.

Tesco said that it tailored its Clubcard Prices loyalty offers at smaller stores as customers tended to buy for different occasions there.

Which? collected data from a total of 11 convenience stores and nearby supermarkets around the country, from Glasgow to Sevenoaks in Kent.