A food shop at a convenience store could be costing you up to a fifth more than buying the same products at larger branches of the same supermarket, new research from consumer group Which? has found.

Which? compared the cost of a basket of 42 popular grocery items, both branded and own-label, on three different occasions in June and July 2024 at the largest supermarket convenience chains — Morrisons Daily, Sainsbury’s (SBRY.L) Local and Tesco (TSCO.L) Express — against prices at their full-sized supermarkets.

Items included in the basket ranged from cheese and pasta to blueberries and ice cream.

The researchers also compared prices for shoppers with loyalty cards — Morrisons More, Sainsbury’s Nectar and Tesco Clubcard — which offer deals to scheme members. However, Which? highlighted that both Sainsbury's and Morrisons do not currently offer scheme member discounts at their convenience stores. This meant that the total average cost of this 42-item grocery basket at these smaller stores was the same, regardless of membership.

Morrisons was found to have the biggest price difference, with the basket of 42 items costing ￡119.29 on average at its convenience store, which was typically just over ￡16 more than if this shop was done at with its loyalty card at a larger branch.

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A 400g tin of own-brand chickpeas cost ￡1 in a Morrisons Daily convenience store — just over double the 49p you'd pay in its larger supermarket.

Meanwhile, a 165g tub of Philadelphia soft cheese would set you back ￡2.58 on average in a Morrisons Daily store, compared to ￡1.58 at a large supermarket branch.

At Sainsbury's Local stores, the basket of groceries cost ￡111.83 on average, nearly ￡11 more than the sane items bought with a Nectar card in its bigger stores.

For Clubcard members shopping at a Tesco Express, the products cost ￡108.28 — ￡10 more than if doing this shop in the chain's larger stores. Without a Clubcard, this shop at a Tesco Express would cost ￡117.30 on average, which was ￡10.50 more than shopping as a non-member in its bigger stores.

Essentials such as, milk, bread and butter were found to have big markups in smaller supermarket branches. All three supermarket convenience stores charged 8% more for two pints of own-label, semi-skimmed milk, with or without a loyalty card.

However, a handful of products were found to be cheaper in convenience stores than in the larger branches.

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For example, Kenco Millicano instant coffee was 32% cheaper at a Morrisons Daily. Meanwhile, Clubcard holders paid 14% less, on average, for a five-pack of Nature Valley cereal bars at Tesco Express.