Tony Blair has failed to exert any influence over American foreign policy and his decision to join the invasion of Iraq will prove a terrible, lasting mistake, one of Britain's most respected think tanks concluded today.

The biting analysis came as American newspapers reported a growing row between the White House and military officials over what to do next in Iraq. The Pentagon reported yesterday that violence had reached the highest levels for 18 months and conditions suggested that Iraq was heading for civil war.

In his examination of ten years of Mr Blair's foreign policy, Victor Bulmer-Thomas, the outgoing director of Chatham House, said that the Prime Minister had offered the White House tremendous political and military support and won very little in return.

"The root failure of Tony Blair’s foreign policy has been its inability to influence the Bush administration in any significant way despite the sacrifice — military, political and financial — that the United Kingdom has made," he wrote.

"Tony Blair has learnt the hard way that loyalty in international politics counts for very little."

Mr Bulmer-Thomas judged Mr Blair's early forays into international politics during the Clinton Administration a "qualified success", saying the Prime Minister balanced a strong relationship with the US and a growing closeness to the EU. But the September 11 attacks proved a decisive moment, and persuaded Mr Blair to side with Bush Administration on Iraq.

"The post-9/11 decision to invade Iraq was a terrible mistake and the current debacle will have policy repercussions for many years to come," the paper said, saying it remained unclear whether the primary justification for the war, intelligence showing Iraq's possession of weapons of mass destruction, were "overblown or even fabricated".

Margaret Beckett, the Foreign Secretary, came swiftly to Mr Blair's defence, saying "the whole basic thesis of this note is just plain wrong". She told BBC Radio 4 that the Prime Minister retained a large influence and that was why he was in the Middle East now, trying to coax the Israeli and Palestinian leaders to the negotiating table.

"When it comes to the governments, the negotiators, the people who are trying to do deals, the people who are trying to bring things together, the people who want advice and support and so on, Tony Blair’s influence continues to be substantial," she said.

The questioning of Mr Blair's transatlantic clout comes at a delicate moment in the formation of America's strategy in Iraq.

Robert Gates, a former head of the CIA, was sworn in as America's 22nd Secretary of Defence yesterday and his first task will be to enter what is being described in Washington as an "intense debate" between the White House and US military officials about the next step to take in the conflict.

The Washington Post reported today that the Joint Chiefs of Staff, the leaders of the various branches of the American armed forces that advise the President on military policy, are unanimously opposed to a White House plan to send an extra 15,000 to 30,000 troops to Iraq for the next eight months.

The military officials say a small surge in US troop numbers will only exacerbate levels of violence in the country, the newspaper reported, without giving the coalition or Iraqi soldiers a decisive advantage over the sectarian militias and militant groups that are driving the fighting.

"The Joint Chiefs think the White House, after a month of talks, still does not have a defined mission and is latching on to the surge idea in part because of limited alternatives, despite warnings about the potential disadvantages for the military, said the officials," the newspaper reported.

"The chiefs have taken a firm stand, the sources say, because they believe the strategy review will be the most important decision on Iraq to be made since the March 2003 invasion."

The Pentagon reported yesterday that violence in Iraq has reached the highest levels since it started presenting quarterly reports on the country's progress to Congress last year.

Attacks against American troops, their Iraqi comrades and civilians reached 959 per week in the period from August to November, up 22 per cent on the previous three months.

The increase means that 93 Iraqi civilians are killed or injured every day in Iraq, while 33 Iraqi security staff are killed or hurt. An average of 25 US and coalition soldiers were killed or injured every day in the same period, just short of the 2004 high, during the assault on Fallujah, the bloodiest period of the war for US troops in Iraq.

The Pentagon said that the level of sectarian violence meant that "conditions that could lead to civil war do exist" in Iraq and identified the Mahdi Army, the Baghdad-based Shia militia, as more dangerous than al-Qaeda in Iraq in the current climate of tit-for-tat ethnic killings.

"The violence has escalated at an unbelievably rapid pace," said Lieutenant General John F. Sattler, director of strategic plans and policy for the Joint Chiefs of Staff, at a briefing for journalists. "We have to get ahead of that violent cycle, break that continuous chain of sectarian violence."

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