The study, produced by the Johns Hopkins Bloomberg School of Public Health in Baltimore and published online by the Lancet, claims the total number of deaths is more than 10 times greater than any previously compiled estimate.

The findings provoked an immediate political storm. Within hours of its release, George Bush had dismissed the figures. "I don't consider it a credible report," he told reporters at the White House. "Neither does General Casey [the top US officer in Iraq], neither do Iraqi officials."

The Foreign Office also cast doubt on the findings, stating that the government preferred to rely on the body count of the Iraqi ministry of health, which recorded just 7,254 deaths between January 2005 and January 2006.

But the US researchers have the backing of four separate independent experts who reviewed the new paper for the Lancet. All urged publication. One spoke of the "powerful strength" of the research methods, which involved house-to-house surveys by teams of doctors across Iraq.

The Johns Hopkins researchers published an earlier study in the Lancet in October 2004, which caused similar shock waves. They say the new work validates the old and shows an alarming escalation in violent deaths.

Nearly a third of the deaths (31%) were ascribed to the coalition forces. Most of the deaths - 601,000 out of 655,000 - were due to violence and of those, 56% were caused by gunshot wounds. Air strikes, car bombs and other explosions accounted for a further 13-14%.

For reasons involving their own safety, the doctors did not probe whether those who died were combatants or civilians. Deaths due to disease have also risen as the conflict has damaged Iraq's health services.

The authors say their discovery that the death rate in Iraq has more than doubled from 5.5 per 1,000 a year before the invasion to 13.3 per 1,000 a year since "constitutes a humanitarian emergency".

"Although such death rates might be common in times of war," write the authors, Professor Gilbert Burnham and colleagues, "the combination of a long duration and tens of millions of people affected has made this the deadliest international conflict of the 21st century and should be of grave concern to everyone.

"At the conclusion of our 2004 study we urged that an independent body assess the excess mortality that we saw in Iraq. This has not happened.

"We continue to believe that an independent international body to monitor compliance with the Geneva conventions and other humanitarian standards in conflict is urgently needed. With reliable data, those voices that speak out for civilians trapped in conflict might be able to lessen the tragic human cost of future wars."

Yesterday the Foreign Office repeated the government's criticism of two years ago. "We will be looking at it in more detail but it is a fairly small sample they have taken and they have extrapolated across the country," said a spokesman.

"We rely on the Iraqi government themselves. They are producing their own figures these days. Our position at the moment is that whatever figures we see, all these civilian deaths are a tragedy and of great concern to us. The multinational forces and the international community have to support a democratically elected government which is trying to stamp out the violence."

The US defence department said that it always regrets the loss of life anywhere. "The coalition takes enormous precautions to prevent civilian deaths and injuries," said its spokesman, Mark Ballesteros.

"By contrast, the enemy in Iraq takes no such precautions and deliberately targets innocent civilians.

"It would be difficult for the US to precisely determine the number of civilian deaths in Iraq as a result of insurgent activity. The Iraqi ministry of health would be in a better position, with all of its records, to provide more accurate information on deaths in Iraq."

The Lancet editor, Richard Horton, says in a commentary published online with the study that the work "corroborates the impression that Iraq is descending into bloodthirsty chaos".

Plans by the Americans to reduce the number of troops in Iraq appeared yesterday to have been scuppered by the growing violence in the country.

General Peter Schoomaker, the US army chief of staff, said he was planning for troop numbers to stay at the present level through to 2010. "This is not a prediction that things are going poorly or better. It's just that I have to have enough ammo in the magazine that I can continue to shoot as long as they want us to shoot," he said.

There are 141,000 American troops in Iraq, and the US government had hinted it would begin reducing numbers to 100,000 after the inauguration of the Iraqi government. But these plans appear to have been jeopardised by increased insurgent attacks and sectarian killings.

Yesterday, Jan Egeland, the UN under-secretary for humanitarian affairs, warned: "Revenge killing seems to be totally out of control" and added that the "blunt, brutal violence" in Iraq was targeting all civilians.

But despite Dr Horton's bleak assessment, he writes, "absolute despair would be the wrong response. Instead, the disaster that is the west's current strategy in Iraq must be used as a constructive call to the international community to reconfigure its foreign policy around human security rather than national security ... Health is now the most important foreign policy issue of our time."