Research by the RAC has shown the “shocking” disparity between the prices of the two fuels, despite wholesale prices being on a par.

RAC Fuel Watch found that the average price of a litre of petrol on UK forecourts now stands at 146.63p while diesel is 164.26p despite both fuels selling for around 114.5p on the wholesale market. In fact, on two days last week wholesale diesel was cheaper than petrol.

Since the start of March, the average weekly wholesale price of diesel has fallen 5p a litre (from 119p to 114.5p) while the price of unleaded petrol has remained the same (114.6p to 114.7p).

Simon Williams, RAC fuel spokesman, said: “The forecourt price disparity between petrol and diesel across the UK is absolutely shocking given their wholesale prices are now virtually identical.

“At the beginning of March wholesale diesel was only 6p more expensive than petrol yet there was a 20p a litre gap between both fuels on the forecourt. Now the two fuels are identical on the wholesale market, and there’s still more than 17p difference at the pump.

“For retailers to be taking a margin of nearly 20p a litre on average throughout March, compared to the long-term average of 7p, is devastating for every driver and business that relies on diesel.”

The RAC said that the price of a litre of diesel should have already come down to around 152p, but now the wholesale price is the same as petrol at 114p we should be seeing forecourts prices of 147p.

One chain bucks the trend

But this seems unlikely given current retailer behaviour, with the big four supermarkets, which dominate UK fuel retailing, charging an “outrageous” 162p a litre for diesel on average.

Williams added: “As the supermarkets buy so frequently they have had plenty of time to pass on the lower prices they are benefitting from on the wholesale market to drivers at the pumps, but they remain totally resolute in their refusal to cut their prices substantially which is nothing short of scandalous, particularly in a cost-of-living crisis.

“The sole national retailer prepared to buck this trend appears to be membership-only chain Costco, which is charging just under 150p a litre for diesel at the moment.”

The RAC said that supermarket petrol station fuel prices are usually around 4p cheaper than the UK average, meaning customers should have been seeing prices under 150p weeks ago.

However, many independent petrol retailers are now charging far less than their supermarket rivals.

“If smaller retailers can afford to make ends meet with lower margins and smaller sales volumes, then what excuse can the supermarkets possibly have for keeping their diesel prices so high?” Williams said.

“We hope the Competition and Markets Authority, which is currently reviewing the road fuel market in the UK, is keeping a watchful eye on this pricing behaviour as we believe it’s against the interests of diesel drivers up and down the country.”