Asda’s fuel has become the priciest among supermarket groups – after years of being among the cheapest – despite its pledge to offer the best value, research suggests.

According to the RAC, average petrol and diesel prices were higher at Asda at the end of May than at Tesco, Morrisons and Sainsbury’s.

Asda disputed the findings, saying they covered its whole estate, including convenience shops, and that it still had the best prices at supermarket forecourts.

Global fuel prices are falling, but the RAC said supermarket chains were not passing the savings on to customers.

The RAC analyed data submitted by the biggest supermarkets to the Competition and Markets Authority and found that Asda was selling petrol at 146.34p per litre and diesel at 151.71p.

By contrast Tesco, Morrisons and Sainsburys were charging 144.45p and 149.61p per litre – some 1.9p and 2.1p less.

RAC senior policy officer Rod Dennis said it was the first time since the motoring group began tracking supermarket fuel prices that Asda had come out as priciest.

He added that the chain had for years been the market leader on fuel prices, encouraging its rivals to drop their prices after it did, but that it had now lost its “crown”.

Mr Dennis added that with fuel prices falling, Asda was unlikely to regain its position any time soon, as rivals were likely to lower their prices just as quickly to maintain their lead.

Asda disputed the findings, saying that prices at its supermarket forecourts specifically remained market leading.

According to its numbers, it sold fuel at its forecourts for 147.78p per litre in May, cheaper than its rivals.

Explaining the higher prices at smaller shops, Asda said that its rates were in line with local market competition.

“Asda was the price leader in the supermarket fuel sector in May and we remain focused on providing our customers with the best value at the pumps as we grow in the convenience sector,” a spokesperson said.

The supermarket chain doubled down on its claim to offer the cheapest fuel prices after it was bought out by the Issa Brothers’ EG Group last year.

Average fuel prices in the UK have been climbing again due to rising global oil prices, although they are expected to fall slightly in the coming weeks.

The RAC said wholesale fuel prices were already coming down but that this was not being reflected at the pumps.

“With only a few exceptions, there doesn’t appear to be much in the way of price competition taking place.”