Fuel retailers are being urged by the RAC to give millions of diesel vehicle drivers an early Christmas present by dropping prices to the same level as those of petrol.
Lower wholesale prices have resulted in the widespread sale of petrol for less than £1 per litre.
But the RAC says that with diesel wholesale prices being 3p per litre cheaper than petrol, motorists should not be paying more for it on the nation's forecourts.
The motoring organisation says despite wholesale diesel being cheaper than petrol, the average price at the pump is currently 107.72p per litre compared to petrol's 104.37p a litre.
Simon Williams, spokesman for RAC Fuel Watch , says: "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.
"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."
Some 74 million litres of diesel are sold in the UK each day compared to 46 million litres of petrol.
Copyright Press Association 2015. Motoring News articles do not reflect the RAC's views unless clearly stated.