Asda has been named the most expensive UK supermarket to buy fuel from, despite its owners pledging that it would be the cheapest grocer on this front.

According to analysis by motoring organisation RAC using data gathered by the Competition and Markets Authority (CMA), Asda charged an average 2.1p per litre more for unleaded petrol than Tesco, Sainsbury’s and Morrisons at the end of May, and this rose further for diesel, with prices as 2.5p per litre more.

RAC senior policy officer Rod Dennis said: “It’s interesting to see that Asda no longer holds the crown for selling the cheapest fuel, despite the pledge made when it was subject to a merger a year ago.”

“The other three major supermarkets, as well as some enterprising independents, now offer lower prices.”

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Last year, the CMA said that Asda’s £600m takeover of 132 Co-op petrol stations “could increase prices” or mean less choice for consumers as it raised competition concerns.

However, in its defence, Asda told the CMA that competition concerns would not arise in these areas because the merger would allow it to bring its low-cost pricing model to more customers.

Asda told The Guardian that it was still the cheapest fuel retailer at its larger supermarket-branded forecourts, however said that it did charge higher prices at its convenience stores due to the market being different.

It claimed that if the c-store forecourts were not included in the analysis, the average price that it offered for unleaded fuel would total 145.12p, compared with Morrisons at 145.17p and Tesco with an average of 145.52p.

An Asda spokesperson said: “We remain focused on providing our customers with the best value at the pumps as we grow in the convenience sector. In May, we reduced the price of unleaded and diesel by a combined average of 2.31p per litre.”

It comes after the CMA set out a voluntary scheme last July, asking supermarkets to release their fuel pricing information to give customers better comparison data on local rates.

At the time, the competition watchdog found that drivers obtaining fuel from the UK’s four main supermarket-owned forecourts had paid an additional £900m for fuel in 2022, as a result of the retailers having hiked their profit margins.

Following this, Asda became the first supermarket to publish local fuel prices online last August, allowing motorists to check the price of petrol or diesel before they travel to one of the supermarkets filling stations.