Diesel drivers are paying around 17p per litre more for a litre of fuel than those with petrol vehicles, despite the wholesale cost being the same.

It means retailers are making almost three times the margin on every litre of diesel they sell, according to figures from RAC Fuel Watch.

The average price of petrol across the UK stands at 146.63p while diesel is 164.26p despite both fuels selling for around 114.5p on the wholesale market.

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

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.  

“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 price of a litre of diesel should have already come down to around 152p, and now the wholesale price is the same as petrol at 114p we really should soon be seeing forecourts displaying prices of 147p. Sadly, this seems unlikely given current retailer behaviour.”

The big four supermarkets, which dominate UK fuel retailing, are charging 162p for a litre of diesel, on average. Williams labelled this “outrageous”.

He 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.”

Costco has bucked the trend and is charging just under 150p a litre for diesel, at the moment.

A number of independent retailers are also charging far less than their supermarket rivals, which is a sign of how much fuel retailing has changed.

“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,” Williams said.