Asda and Morrisons have both cut the cost of petrol to under £1 a litre, raising the stakes in the Christmas price war.
The Asda price drop, falling by 2p to 99.7p, will last three days from today, while Morrisons said it hopes to keep its everyday price of 99.9p for "as long as possible".
Asda said the price of diesel will also be cut to 103.7p per litre, while Morrisons will cut its by 1p.
An Asda spokesman said the promotion, due to run at all 277 of its filling stations from Friday to Sunday, reflects the savings it has made thanks to the price of unleaded hitting a six-year low.
It is the second time this week the retailer has cut the price of petrol to less than £1.
Andy Peake, Asda's senior petrol director, said: "We're adding a further boost by investing in a three-day fuel price drop meaning drivers can now benefit from fuel as low as 99.7ppl in the crucial run-up to the festive period.
"We know our competitors are likely to follow but Asda don't have a postcode lottery on fuel, meaning no matter where customers live, they will benefit from the same fuel price at every single one of our filling stations with our national price cap of 99.7ppl for unleaded and 101.7ppl for diesel."
Bryan Burger, petrol retail director at Morrisons, said: "Today, for the first time in more than six years, we are moving unleaded prices down to below £1-a-litre.
"This is a moment where motorists will feel some relief after being clobbered by tax and price rises for the last decade."
He claimed the average car driver will now save £17 a week since the petrol price peak of May 2011.
The retailer said it would keep the low price for as long as possible but added "it remains subject to changes in oil markets and foreign exchange movements".
The price per barrel of Brent crude dipped below 40 dollars earlier this week and the average price of diesel is already at a six-year low, dropping to 109.18p last Friday.
An Asda spokesman said the temporary fuel price cap was not a loss leader but part of its customer-led festive pricing regime that began with its decision to pull out of Black Friday.
He said: "In November we pulled out of Black Friday after we got so much feedback from our customers to invest in other festive products and one of those was fuel. It's just one of many products we have looked at as part of our price investment."
The AA said the temporary fall will bring Asda's petrol price below the £1 mark familiar to drivers on the continent and across the Irish Sea.
The latest figures show the price of unleaded at 83.4p in Spain and 93.2p in Ireland at the current exchange rate, while Spanish drivers pay just 74.57p for their diesel and Irish motorists 86.05p.
RAC Fuel Watch spokesman Simon Williams said: "Another promotional cut to enable motorists to buy petrol at under £1 will no doubt bring early Christmas cheer to many, but we would like to see this price being held for longer than three days with oil under 40 US dollars a barrel and the wholesale price of unleaded down.
"As we have said this week, we believe that petrol for £1 or under will be a common sight in the run-up to Christmas."