Leading article: This shocking figure reveals the true cost of the war

Numbers have been a source of contention in coverage of the Iraq war since well before the US and British invaded Iraq. The number of troops that would be required, not just to remove Saddam Hussein, but to secure the country afterwards, was a matter of debate, although few now dispute it was far too few to accomplish the mission successfully. The quantity of lethal weapons and materiel that Saddam was said to possess was the subject of equally fierce argument.

And while the US and British military authorities have been meticulous in releasing figures for the number of their own soldiers killed, they have been not been nearly so precise about counting the wounded, prompting accusations from servicemen's charities that they had sought to conceal the real scale and gravity of casualties. As for the number of Iraqis killed and wounded in the past three and a half years years - surely a key measure of the cost of the war to the supposed beneficiary of the invasion - the US and British authorities have until very recently remained scandalously silent.

Having decided, so the US authorities said at the outset, it would be too difficult to log figures for Iraqi casualties with any accuracy, the Pentagon started slipping out estimated numbers of Iraqi dead a year ago. Last December, President Bush gave a figure of about 30,000 for Iraqi civilian deaths. This was below the then estimate given by the Iraq Body Count project, which has been compiling figures from news reports and eye-witness accounts since 2003, but the discrepancy was not enormous. At least not when set against the number of Iraqi losses estimated by a US university medical study two years ago.

This study, reported - and by implication validated - by the British medical journal, The Lancet - argued that Iraqi casualties had been grossly underestimated. Applying a different methodology, which relied on household sampling, rather than news and other reports, the epidemiologists from Johns Hopkins University, extrapolated a figure of 100,000 Iraqi lives lost in the first 18 months after the invasion. So shocking was this figure that it was widely dismissed in official circles as unreliable or hyped for political reasons. This same group has updated its study, using similar methodology but with a larger sample. This has produced an estimated Iraqi death toll in the war to date of a catastrophic 655,000. This is more than 13 times the current Iraq Body Count figure of between 44,000 and 49,000.

Some part of the discrepancy can be explained by the fact that like is not being compared with like. The IBC figures are based on reports of actual deaths clearly linked to the conflict. The Johns Hopkins study, again reported in The Lancet, uses the results of its sampling to project numbers of deaths across the whole country; it then compares this figure with a figure based on death rates before the war. Its definition of war-related deaths is, therefore, rather broader, in that includes people whose deaths may be linked to the conflict less directly. The availability of guns, the shortage of medical staff, the increased incidence of disease due to poor hygiene, and other, perhaps psychological, reasons must all contribute to Iraq's soaring higher death rate.

We fully expect this latest study to become as great a source of political contention as its predecessor. Some will conjure up a picture of 655,000 Iraqis - 2.5 per cent of the population - gunned down in combat; others will denounce the figures as hyped by opponents of the war. A more sober judgement would be that, while the new figures contain an element of hypothesis, the true cost of the war in Iraqi lives is vastly higher than the invading countries have estimated. And the damage to Iraq's long-term future is higher still.