The average cost of a litre of petrol fell more than 4p over the weekend, with diesel prices also plummeting.
On Friday, petrol averaged 118.33p a litre - by Monday that had dropped to 114.01p. Diesel fell from 120.41p to 118.31p a litre after the weekend.
That's the biggest fall this century according to the AA, overtaking the 2.61p recorded in October 2008 during the financial crash.
AA fuel price spokesman Luke Bosdet said: "Pump prices are beginning to reflect the plunge in costs but there is much further for them to fall.
“More retailers are pricing their petrol at £1 a litre, with a fuel station near Yorkgate in north Belfast reported to have joined their ranks over the weekend.”
Oil prices continued to fall on Monday, hitting 18-year lows trading at just $22.23 a barrel yesterday.
Economist Intelligence Unit analyst Cailin Birch said: "The floor has dropped out of the oil market, and we do not expect it to return until the fourth quarter."
The AA pointed out the cost of wholesale petrol dropped to an average of 16.09p a litre last week and, yesterday, reached 15.69p.
Once tax and a 9p margin for the supplier and retailer is added in - that points to a pump price of 99.54p a litre.
But Bosdet had a word of warning for drivers.
“Lower pump prices are not an excuse for people with cars to break the lockdown and drive unnecessary journeys. But they do offer some welcome news to lift drivers’ spirits,” he said.