The call comes from the RAC, which says the average diesel price should reach £1 if retailers pass on wholesale cost-savings and is urging the fuel retailers “not to take advantage of motorists by charging more for diesel than they should”.

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, which monitors the same data as retailers use to inform their buying decisions, 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. While the wholesale price of diesel is currently 3p lower per litre than petrol, the average price at the pump was 3p more expensive at 107.72p on Monday 14th December compared to petrol at 104.37, which dropped almost 2p since Friday as a result of the sub-£1 litre.

Simon Williams added: “Our Fuel Watch data shows the average price of diesel should come down by about 6p to around £1 from its current average price which indicates that the wholesale cost-savings haven’t been passed on quickly enough.”

The RAC added that the long-term outlook is for the oil price to stay low.