The wholesale cost of diesel is now approximately the same as the cost of petrol, yet on average the price on UK forecourts is 6p-per-litre higher for diesel than it is for petrol.

In late May the wholesale cost of petrol was 103.5p, while the cost of diesel was just 0.5p more.

The RAC is calling on the fuel industry to explain why the reduced wholesale costs aren’t being passed on to customers in the form of cheaper pump prices.

It is hoped that this would help take some of the pressure off cash-strapped motorists in the UK.

The RAC official said that, although the price is petrol increases has eased, it has still “never been a more expensive time to be a motorist”.

RAC fuel spokesman Simon Williams said: “Transparent, fair fuel pricing is vital for the economy and to maintain the trust of motorists.”

“While two thirds of Britain’s 29m cars run on petrol we use twice as much diesel, around 26bn litres a year, which demonstrates how important it is to business through our 460,000 HGVs, 3.2m vans and all the diesel-powered company cars on the road which cover high mileages every year.”

“In the last year retailers have been noticeably quicker to pass on reductions in the wholesale price to forecourts, but we are now seeing an unfortunate blip in that trend where diesel prices are higher than they really need to be.”

“There has been talk of the fuel retailers using higher diesel prices to keep petrol prices lower, but whether or not that’s the case, the simplest way to operate must be to make sure retail prices always reflect wholesale prices proportionately. This has to be fairer in the long run for both private motorists and businesses alike.”