The RAC has accused the big four supermarkets of outrageous behaviour over the pricing of diesel as its wholesale price is now on a par with petrol yet diesel is still being sold for 17.5ppl more on average at UK forecourts.

It also noted that many independent forecourt operators are selling diesel for far less than their supermarket rivals.

The average price of petrol is 146.63ppl while diesel is 164.26ppl, but the RAC says both fuels are selling for around 114.5ppl on the wholesale market, and 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 5ppl while unleaded has remained the same (diesel 119ppl to 114.5ppl; petrol 114.6ppl to 114.7ppl).

RAC fuel spokesman Simon Williams 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 6ppl more expensive than petrol yet there was a 20ppl 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 17ppl difference at the pump.

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

“The price of a litre of diesel should have already come down to around 152ppl, and now the wholesale price is the same as petrol at 114ppl we really should soon be seeing forecourts displaying prices of 147ppl. Sadly, this seems unlikely given current retailer behaviour. Instead, the big four supermarkets, which dominate UK fuel retailing, are charging an outrageous of 162ppl on average.

“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 150ppl for diesel at the moment.

“Since the beginning of March, they have only reduced diesel by just over 3ppl. Considering supermarket prices are usually around 4ppl cheaper than the UK average, their customers should have been seeing prices under 150ppl weeks ago.

“We are also seeing many independent retailers charging far less than their supermarket rivals which is a sign of how much fuel retailing has changed. This would have been pretty unusual several years ago but is now rapidly becoming the norm. 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?

“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.”