The RAC is urging supermarkets to cut their diesel prices to those of petrol, or even below, because wholesale prices of diesel are 3ppl less than petrol.

It warns that if they don’t “they are in danger of being seen to take the country’s 11m diesel car drivers and Britain’s 3m-plus ‘white van’ men for a ride”.

RAC Fuel Watch spokesman Simon Williams said: “We hope the initial tight margins involved in selling petrol for under £1 haven’t led retailers to try to make up their lost profit by pricing diesel higher than it should be.

“Even though we saw the price of diesel drop below that of petrol on many forecourts a few months ago, there must be a temptation for retailers to maintain the differential and take advantage of motorists who have – over a period of years – become used to paying more for diesel than unleaded at the pump.

“As a result of two new refineries in Saudi Arabia coming on-stream, wholesale diesel is more widely available than ever before, which means the price should be far cheaper going forward than in previous years. This, coupled with an oil price that has been below $40 a barrel for the last week, and still appears to be falling, should make it possible for diesel to be sold for under £1 a litre by the vast majority of retailers.”

RAC Fuel Watch reports that the wholesale price of diesel was below that of petrol all last week and was at a similar level to petrol the week before. Meanwhile the average price at the pump was 3p more expensive at 107.72ppl on Monday 14 December compared to petrol at 104.37.

Simon Williams added: “After breaking through the $40 mark, the price of a barrel of Brent Crude has continued to drop and stood at $36.81 when the markets closed on Monday, a price last seen in December 2008. Similarly, the pound has made up some of the ground it lost on the dollar in the last fortnight so wholesale prices are continuing to stay low, and in the case of diesel has fallen. This means we should see petrol stay around £1 a litre for a while, with diesel joining it if retailers are fair to motorists and pass on the savings.”