WASHINGTON — The war in Iraq has boosted Islamist terrorism, and the threat to the West has increased since the September 11, 2001, attacks, according to leaks from a report by America’s intelligence agencies.

In the latest blow to the justification made by President Bush and Prime Minister Blair for March 2003’s pre-emptive invasion, the National Intelligence Estimate has concluded that the war has fueled violent extremism and spawned a new generation of militants.

A collation of work from America’s 16 spy agencies, the report is the first official survey of American intelligence on global terrorism since the Iraq invasion.

Critics of the war — who have long argued that it is an effective recruiting sergeant for Islamist militants — seized on summaries of the study, which were leaked to the New York Times yesterday.

A spokesman for the White House, which has persistently argued that the world is a safer place because of the war, would only say that the leaks did not give a balanced account of the report.

Called “Trends in Global Terrorism: Implications for the United States,” the estimate argues that Islamic extremism has spread across the world and diversified, according to the leak.

An early chapter — “Indicators of the Spread of the Global Jihadist Movement” — highlights the Iraq war as a prime cause for the spread of the ideology of violent holy war. The 30-page estimate cites the “centrality” of the American-led invasion and the ensuing insurgency as the inspiration for Islamist terror networks across the world.

“It’s a very candid assessment,” one intelligence official told the Washington Post. “It’s stating the obvious.”

The report says Al Qaeda has been disrupted since the September 11, 2001, attacks, with its leadership scattered and on the run since the overthrow of the Taliban in November 2001.

But it concludes that the worldwide Islamist movement has expanded from a core of Al Qaeda operatives to include a new class of “self-generating” cells motivated by Osama bin Laden but without any direct connection to him. Senator Kennedy, a Democrat of Massachusetts, said the report should “put the final nail in the coffin for President Bush’s phony argument about the Iraq war.”

This is not the first time that a National Intelligence Estimate has sparked contention. The Bush administration drew on one drafted in autumn 2002 to make its case for war. That report concluded that Iraq “probably will have a nuclear weapon during this decade.”