Supermarket convenience stores could cost shoppers up to a fifth more than the same grocery products in the retailer’s full-sized stores, a new study from Which? has revealed.

The consumer group conducted a study comparing the price of 42 grocery products over three separate occasions in June and July at convenience stores, Morrisons Daily, Sainsbury’s Local and Tesco Express, to the supermarket chains’ larger stores.

It found the greatest price difference between branded and own-label products at Morrisons, with customers shelling out 21% more on average for the same items at its Daily stores.

Meanwhile, members of Morrisons’ More loyalty scheme could also face prices up to 22% more at its convenience branches 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”.

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Elsewhere, the study found Tesco shoppers can expect to pay a 10% price difference and its Clubcard holders a 11% difference at its convenience outlets, while Sainsbury’s customers face a 5% difference at its Express stores and Nectar members could find products 14% higher.

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

Both Tesco and Sainsbury’s said that their smaller stores were often in city of town centre locations, where their price reflected higher rents, rates and other operating costs.

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.”

The Which? study follows a series of reports it has recently launched into supermarket prices. Last month, the consumer watchdog challenged grocery retailers’ loyalty schemes, with an investigation that cautioned the initiatives offered “questionable” discounts.