Fuel retailers have been urged to cut the price of diesel to under £1 per litre.
Unleaded fell below that symbolic level at many supermarkets last week for the first time since 2009, excluding promotions.
But diesel remains more expensive despite its wholesale price being around 3p per litre (ppl) cheaper than unleaded.
RAC Fuel Watch spokesman Simon Williams said oil’s fall in price to a near seven-year low combined with a boost in supply “should make it possible for diesel to be sold for under £1 a litre by the vast majority of retailers”.
He went on: “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.”
Despite last week’s cut in supermarket fuel prices, the average across all retailers is 104.37ppl for unleaded and 107.72ppl for diesel.
AA spokesman Luke Bosdet commented: “The supermarket fuel price war has continued its ferocity beyond the weekend, leaving the nearest non-supermarket retailer trailing on average 5ppl in their wake.
“In the past, supermarkets’ fuel cost savings would have been passed on as money off fuel vouchers based on how much customers spent in store.
“This Christmas the savings are available to everyone – a true offer in the festive spirit.”