Shopping at local convenience stores can cost up to a fifth more than buying the same items at the supermarket chains’ larger stores, according to a new study from Which?.

The consumer group compared the price of 42 popular grocery items on three occasions in June and July at Morrisons Daily, Sainsbury’s Local and Tesco Express compared to those at the supermarkets’ full-sized stores.

The biggest price difference for the branded and own-label items was at Morrisons, with shoppers paying 21% more on average for the same products at its Daily stores.

Members of the supermarket chain’s More loyalty scheme could be charged up to 22% more at the smaller outlets because the scheme is not currently available there.

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

Meanwhile, the average price difference at Tesco was 10% and 5% at Sainsbury’s with the price difference far greater for those signed up to the retailers’ loyalty schemes.

Tesco Clubcard members could pay 11% more in the retailer’s Express stores compared to its full-size branches, while Sainsbury’s Nectar users could find items 14% pricier.

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

Which? retail editor Ele Clark 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.”

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