Brent oil slumped Thursday to a new four-year low on expectations that OPEC will be unlikely to cut production levels to counter the global supply glut, traders said.
At about 0820 GMT, London's Brent crude for delivery in December dived to $79.55 per barrel, hitting a low point last witnessed in late September 2010.
The contract later stood at $79.88 per barrel, down 50 cents from the closing level on Wednesday, when it had first tumbled below the psychological $80 barrier.
"Brent crude (is) hitting a new four-year low on continued concerns that OPEC will fail to address a rising supply glut," Saxo Bank analyst Ole Hansen told AFP.
US benchmark West Texas Intermediate for December delivery meanwhile lost 45 cents to $76.73 a barrel.
The Brent price was also affected by technical reasons as speculative traders sold off ahead of Thursday's expiry of the December contract.
Dealers remain pessimistic over the possibility of the 12-nation Organization of the Petroleum Exporting Countries (OPEC) reducing output to stem a global glut, in part caused by a flood of US "tight oil" extracted from shale.
OPEC's next meeting is set for November 27 in Vienna, home to the cartel's headquarters.
On Wednesday, Saudi Arabia's oil minister rejected talk that the country was leading a price war in global crude markets as prices tumble.
"Talk of a price war is a sign of misunderstanding -- deliberate or otherwise -- and has no basis in reality," Ali Al-Naimi told a conference in Acapulco, according to the text of his speech.
Since mid-June, Brent oil prices have also fallen by more than 30 percent in value amid concerns over abundant supplies.
In addition, the oil market was also pushed lower on Thursday by the stronger dollar and worries over slowing demand.
"Brent front month futures breached below $80 per barrel ... suggesting further downside momentum in the oil market," added Sucden analyst Myrto Sokou.
"Brent futures have retreated more than $35 in nearly five months  ... as bearish oil fundamentals and a sharp slowdown in the oil demand from the US and Asia weighed on market sentiment."
Last week Riyadh sent global oil prices plunging when it cut its prices for the US market while raising them for Asia, the country's major outlet.
Analysts guessed that the country wanted to strengthen its market share in the United States against a flood of domestic oil from shale deposits.