The U.S. crude contract fell as low as $41.35 a barrel, the weakest level since early 2009, when the global economy was in the throes of a massive financial crisis and recession. By late morning in Europe, the price was around $42 a barrel.
The oil contract has been declining since touching a high of $61.43 on June 10.
The main reasons are big increases in production in the U.S. and Canada, as well as expectations Iran's oil could soon return to the market. Meanwhile, China's devaluation of its currency this week highlights its concern about an economic slowdown.