An all-out oil war between Saudi Arabia and Russia has led to stock markets and oil prices tanking around the world.
As investors already deal with the fallout from coronavirus, the Middle Eastern oil giant announced plans to flood markets with extra supply – wiping billions off companies.
Motorists will be hoping the decision could see petrol forecourts reduce prices by up to 10p a litre.
However, supermarkets are already stretched with coronavirus preparations and it remains unclear how quickly the cuts will be passed on at forecourts.
The falls started in Asia overnight, spreading to Europe and beyond and then to the US, where the New York Stock Exchange was suspended for 15 minutes following a 7% crash immediately after opening.
In London, companies on the FTSE 100 had lost around £125 billion at the end of the day – adding to heavy losses over coronavirus fears – and means the index has fallen by almost 20% in just over two weeks.
The Saudis are trying to punish Russia after the two sides failed to agree to supply targets. Leaders will also be hoping it pushes US shale gas companies under, as their operations become economically unviable.
The move sent the price of a barrel of Brent Crude down 30% initially, before settling at around 20% down on a day earlier, to just 36.1 dollars a barrel.
The impact was felt across the world, with the FTSE 100 losing 7.7%. It spent most of Monday swinging between falls of 8% and 6%.
The smaller FTSE 250 was also both down by 6.4% with oil companies taking the brunt.
BP, Shell and British Gas owner Centrica were all some of the biggest fallers, with shares dropping between 15% and 20% through the day.
Neil Wilson, chief market analyst at Markets.com, said: "This will be remembered as Black Monday. If you thought it couldn't get any worse than the last fortnight, think again. The blood really is running in the streets, it's utter carnage out there.
"The oil price shock has totally unnerved investors while Italy's decision to quarantine 16 million citizens in the north of the country has left markets feeling like the coronavirus outbreak is out of control – where next? The UK is preparing for the worst."
But it was smaller companies who were hit even harder – Premier Oil and Tullow Oil lost 57.5% and 31.8% of their value respectively.
Seventeen of the top 100 companies lost more than 10% of their value.