The figure for the number of deaths attributable to the conflict - which amounts to around 2.5% of the population - is at odds with figures cited by the US and UK governments and will cause a storm, but the Lancet says the work, from the Johns Hopkins Bloomberg School of Public Health in Baltimore, has been examined and validated by four separate independent experts who all urged publication.
In October 2004, the same researchers published a study estimating that 100,000 Iraqis had died as a result of the war since the beginning of the March 2003 invasion, a figure that was hugely controversial. Their new study, they say, reaffirms the accuracy of their survey of two years ago and moves it on.
"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."
The epidemiological research was carried out on the ground by teams of doctors moving from house to house, questioning families and examining death certificates. Between May and July this year, they visited 1,849 households in 47 separated clusters across the length and breadth of Iraq. The doctors asked about deaths among members of the household in a period before the invasion, from January 2002 to March 2003, and about deaths since. In 92% of cases, they were shown death certificates confirming the cause.
A total of 629 deaths were reported, of which 547 - or 87% - occurred after the invasion. The mortality rate before the war was 5.5 per 1,000, but since the invasion, it has risen to 13.3 per 1,000 per year, they say. Between June 2005 and June 2006, the mortality rate hit a high of 19.8 per 1,000.
Thus they calculate that 654,965 Iraqis have died as a consequence of the invasion. It is an estimate and the mid-point of a range of numbers that could equally be correct in the context of their statistical analysis. But even the lowest number in the range - 392,979 - is higher that anyone else has suggested. Of the deaths, 31% were ascribed to the US-led forces. Most deaths were from gunshot wounds (56%), with a further 13% from car bomb injuries and 14% the result of other explosions.
"Since 2004, and especially recently," writes the Lancet editor, Richard Horton in a commentary, "independent observers have recognised that the security situation in Iraq has deteriorated dramatically." The new study, he continues, "corroborate the impression that Iraq is descending into bloodthirsty chaos".
Yet, 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, around health and wellbeing in addition to the protection of territorial boundaries and economic stability.
"Health is now the most important foreign policy issue of our time. Health and wellbeing - their underpinning values, their diverse array of interventions and their goals of healing - offer several original dimensions for a renewed foreign policy that might at least be one positive legacy of our misadventure in Iraq."