The report sparked heavy criticism by US Democrats, with Senator Kent Conrad, chairman of the US senate budget committee, saying the reliance on contractors "restricts accountability and oversight; opens the door to corruption and abuse and ... may significantly increase the cost to American taxpayers".

The study did not include figures for 2008 and therefore the total paid to
contractors for work in Iraq since the invasion could be even higher, possibly topping $100 billion by the end of this year, the report said.

The US military has relied increasingly on contractors in its wars as it reduced the size of its own forces.

Increased scrutiny

The CBO estimated on Tuesday that between $6 billion to $10 billion has gone to pay for security work in Iraq, which was comparable to the costs of having a US military unit performing the same tasks.

The work of contractors in Iraq - of whom between 25,000 to 30,000 are in security work - has faced increased scrutiny following a series of fatal incidents involving Iraqi civilians.

Last September, 17 Iraqis were killed in a shooting involving contractors from
Blackwater security company, an incident that provoked furious protests from the Iraqi government and strained relations with the US.

The US Justice Department is expected to decide soon whether to bring charges over the incident, although Blackwater itself is not expected to be prosecuted.

The firm recently announced that they planned to scale back their security
contracting business and focus on other areas, due in part to negative publicity from the shooting.

In addition, contracting companies such as Halliburton, previously run by Dick Cheney, the US vice-president, and KBR, formerly known as Kellogg, Brown and Root, have also been accused of overcharging and corruption.

Charles Tiefer, a professor of government contracting at the University of Baltimore Law School, told the New York Times newspaper that the scale of contractor usage in Iraq was "unprecedented".

"It was considered an all-out imperative by the administration to keep troop levels low, particularly in the beginning of the war, and one way that was done was to shift money and manpower to contractors," he told the paper.

"But that has exposed the military to greater risks from contractor waste and abuse."