Prices reached as low as $63.78 per barrel at 5:00 pm GMT on Wednesday, which is a new five-year record low, according to official figures.
WTI, the American benchmark for oil prices, also fell almost 5 percent on Wednesday, reaching nearly $60 per barrel.
In its Monthly Oil Market Report for December, OPEC revised demand for its crude oil in 2015 down by 0.3 million barrels per day.
"Demand for OPEC crude in 2015 is revised down by 0.3 million barrels per day to stand at 28.9 million barrels per day," the report said.
It added that this is 0.4 million barrels per day less than the estimated level for 2014.
The organization met on Nov. 27 to devise a strategy against falling oil prices.
They decided against cutting production to increase prices and stated that they will maintain a production rate of 30 million barrels per day for the first half of 2015.
"Because OPEC did not decrease its production quota, the concern is over-supply of oil next year," said Richard Mallinson, a geopolitical analyst at London-based energy market consultancy Energy Aspects.
The glut of oil in the markets and low global oil demand are regarded to be some of the main factors behind the slump of oil prices since June, when Brent crude was $115 per barrel.
"Demand is weak, and at the same time, supply is rising rapidly," said Thomas Pugh, a commodities economist at Capital Economics, a London-based independent research company.
Pugh stated that he does not expect oil prices to go below $50 per barrel, adding "At that level, you start to get rapid production cuts."
West Texas Intermediate, based in Cushing, Oklahoma, is not only affected by excess supply and low demand in global oil markets, but also by rising oil production in the U.S.
Since the shale boom in 2008, U.S. domestic oil production increased from 1.8 billion barrels in 2008 to 2.7 billion barrels in 2013, according to the U.S.' Energy Information Administration.