Oil prices have fallen below $100 a barrel for the first time in nearly six months after Opec said it would not cut production.
Oil prices have come down a lot since earlier this year
In London, Brent North Sea crude for October delivery plunged $4.14 to $99.30.
In the US, light, sweet crude fell $3.08 to settle at $103.26 on the New York Mercantile Exchange.
Opec president Chakib Khelil said the cartel would keep production unchanged, despite sinking prices and slowing economic growth.
He told reporters at Opec's six-montly meeting in Vienna that prices would stay in the $90-$100 range long term.
Oil prices peaked at a record high of $147 in July - causing inflationary pressure and economic woes across the world.
Read oil price blog by Sky's business presenter Emma Crosby.
Petrol prices on UK forecourts have not fallen as quickly as the oil price, with average unleaded petrol down from July's peak of 119.7p to 112.7p at the weekend - a drop of nearly 6%.
Average diesel prices have fallen from 133.3p to 124.1p, just under 7%.
The AA said it takes around four to six weeks for oil price changes to filter through to forecourt pumps.
Last month, the International Energy Agency forecast that oil demand would contract to 48.6 million barrels per day this year and then fall to 48 million in 2009.
Opec nations have around two-thirds of the world's known oil reserves.
