Asda has sparked a likely supermarket price war by announcing a cut in the cost of its fuel - while oil sector giants BP and Shell have seen their value slip today.
With world oil prices plunging, Asda is knocking 2p a litre off its petrol and diesel from tomorrow.
This means drivers will pay no more than 117.7p a litre for petrol and 121.7p for diesel.
It is the company's seventh cut in two months and means its petrol has gone down by 9p a litre and its diesel by 8p a litre since the end of September.
Asda's petrol trading director, Andy Peake, said: "Asda continues to lead the way on petrol price cuts as we know how important it is to pass savings back into motorists' pockets as soon as oil prices fall."
RAC fuel spokesman Simon Williams said: "As Opec decided not to cut production yesterday, the Brent crude price dipped to 70.68 dollars a barrel at the end of the day, which is the lowest it's been since June 2010.
"This means there will be more oil in circulation than there is demand. The result is excellent news for motorists this Christmas, with the cheapest forecourt prices for four years."
He went on: "But if retailers are truly going to reflect the wholesale price savings, today's cuts should be the first of several in the coming weeks.
"We believe pump prices should fall by well over 4p a litre in the next few weeks, taking the average price of petrol considerably below the 120p a litre mark, which last happened on December 1 2010."
A slump in oil stocks put pressure on the FTSE 100 Index today in the wake of Opec's decision to keep pumping crude at existing levels.
The price of Brent crude stood at around 72 US dollars a barrel - some 37% off its level in June and the lowest since August 2010 - following the conclusion of yesterday's oil cartel meeting in Vienna.
The FTSE 100 Index was 0.7% lower as oil and gas exploration group BG slumped 7% and heavyweight rivals Royal Dutch Shell and BP slipped by around 3%. Glasgow-based oil services company Weir declined almost 7%.
A combination of the US shale boom and weakness in the global economy has resulted in supply outpacing demand, creating pressure for a cut in output to try to staunch the price fall.
IG analyst Alastair McCaig said: "The effects of yesterday's Opec's decision to maintain its oil output is still working its way through the markets today."
The move comes as supermarket Asda said today it will cut up to 2p off the price of unleaded petrol and diesel from Saturday.
At the Opec meeting, Saudi Arabia blocked calls from other members for production cuts to arrest a slide in global prices.
Earlier this month, the Gulf state had even cut the price of oil it sold to the US - seen as an attempt by the kingdom to remain competitive with shale oil.
Some poorer members of the group had pushed for a fall in production to try to drive oil prices back up today. But the impact of the US shale boom meant it was unlikely to have caused a sizeable dent in supply.
The Saudis are able to cope with lower prices while the likes of Venezuela and Nigeria need a level closer to 100 US dollars.
In June, the price of Brent crude reached nearly 116 US dollars as the advance of Islamic State sparked fears over supplies from Iraq.
But gathering gloom over growth - with the eurozone stagnant and Chinese expansion easing - has raised fears of a glut of oil swilling around the world economy.
Brent crude slipped below the 100 US dollar mark in September and has continued to head lower - helpful for petrol-guzzling consumer economies but costly for many oil-producing nations and large oil firms.
In Britain it has helped keep a lid on inflation as the slide feeds through to fuel pump prices though petrol firms face pressure to do more to pass on the lower costs.
Meanwhile, BP and Royal Dutch Shell have acknowledged the impact on their business, causing them to tighten the purse strings rather than pursue some costly spending projects.
BP recently reported a fall in quarterly profits and said it had to maintain a "strictly disciplined approach to investment".
Shell has seen a sharp rise in third-quarter profits but chief executive Ben van Beurden said the "volatility" in prices underlined the need to keep a tight hold on costs and spending as it focused on slimmed-down future plans.