WASHINGTON — The United States has spent $100 billion or more on contractors in Iraq since the invasion in 2003, a milestone that reflects the Bush administration's unprecedented level of dependence on private firms for help in the war, according to a government report to be released today.

The report by the Congressional Budget Office, according to people with knowledge of its contents, will say that one out of every five dollars spent on the war in Iraq has gone to contractors for the U.S. military and other government agencies, in a war zone where employees of private contractors now outnumber American troops.

The Pentagon's reliance on outside contractors in Iraq is proportionately far larger than in any previous conflict, and it has fueled charges that this outsourcing has led to overbilling, fraud and shoddy and unsafe work that has endangered and even killed American troops. The role of armed security contractors has also raised new legal and political questions about whether the United States has become too dependent on private armed forces on the 21st century battlefield.

The budget office's report found that from 2003 to 2007, the government awarded contracts in Iraq worth about $85 billion, and that the administration was now awarding contracts at a rate of $15 billion to $20 billion a year. At that pace, contracting costs will surge past the $100 billion mark this year. Through 2007, spending on outside contractors accounted for 20 percent of the total costs of the war, the budget office found, according to people with knowledge of the report.

Several outside experts on contracting said that the report's numbers seemed to provide the first official price tag on contracting in Iraq and raised troubling questions about the degree to which the war had been privatized.

Contractors in Iraq now employ at least 180,000 people in the country, forming what amounts to a second, private army, larger than the U.S. military force, and one whose roles and missions and even casualties among its work force have largely been hidden from public view. The widespread use of these employees as bodyguards, translators, drivers, construction workers and cooks has allowed the administration to hold down the number of military personnel sent to Iraq, helping to avoid a draft.