The price of crude oil in the United Stated has slumped to a fresh six-and-a-half-year low and once was set to break below $40 per barrel.

US benchmark West Texas Intermediate for delivery in September fell 4.3 percent to $40.80 a barrel in New York Exchange on Wednesday, their lowest close since March 2, 2009.

At one time, the price dropped as low as $40.46 a barrel on the New York Mercantile, following the release of data that showed a huge unexpected stockpile build in the United States.

In London, European benchmark Brent oil for October delivery dropped $1.65 to $46.81 a barrel.

The oil price slump came after the US government's Energy Information Administration said that oil stocks rose 2.6 million last week to 456.21 million barrels.

Traders and analysts expect US oil prices to fall below $40 a barrel with the end of summer driving season.

"As we go into the next couple of months, crude oil demand is going to decline, which worries the market," said Andy Lipow, head of Houston consultancy Lipow Oil Associates.

"If oil is to fall to $40 a barrel, there is nothing to suggest it won’t fall even further," said Fawad Razaqzada, a London-based analyst for crude oil.