Twenty-four experts from the UK, the US, Australia, Canada, Spain and Italy say the attitude of the governments is "wholly irresponsible". They say the UK government's reliance on "extremely limited data" from the Iraqi ministry of health is "unacceptable" because it is likely to seriously underestimate the casualties.
Their hard-hitting statement, published online by the British Medical Journal, comes nearly five months after the Lancet published a household survey of civilian deaths in Iraq which estimated that about 100,000 civilians had died - most of them women and children.
The study caused controversy and was dismissed by the British government as unreliable, partly because the authors admitted that, under the difficult circumstances, it could not be precise.
The experts lambast the government for criticising the data without conducting inquiries of its own. "The obvious answer to removing uncertainties that remain is to commission a larger study with full official support and assistance, but scientific independence," they say.
"Counting casualties can help to save lives both now and in the future by helping us to understand the burden of death, and residual burden of injury, disease and trauma across the entire population," the experts say. "We have waited too long for this information."
The Iraqi ministry of health data is not complete. Among the reasons for this are that only violence-related deaths reported through the health system are counted and deaths in the first 12 months of the conflict are not included.
Among the 10 experts from the UK who have signed the statement are Klim McPherson, visiting professor of public health epidemiology at Oxford University, David Hunter, chair of the UK Public Health Association, and Sian Griffiths, immediate past-president of the faculty of public health at the Royal College of Physicians.
There are seven eminent physicians from the US, three from Australia, two from Spain and one each from Canada and Italy.
"Monitoring casualties is a humanitarian imperative," they say. "Understanding the causes of death is a core public health responsibility, nationally and internationally. Yet neither the public, nor we as public health professionals, are able to obtain validated, reliable information about the extent of mortality and morbidity since the invasion of Iraq."
In a commentary in the BMJ, Professor McPherson says that public access to reliable figures is important. "The policy being assessed - the allied invasion of Iraq - was justified largely on grounds of democratic supremacy. Voters in the countries that initiated the war and others - not least in Iraq itself - are denied a reliable evaluation of a key indicator of the success of that policy. This is unacceptable."
Understanding the burden of death, disease, injury and trauma aids the proper planning of war and health and will help governments assess the humanitarian implications of conflict, he says.
"The plain fact is that an estimate of 100,000 excess deaths attributable to the invasion of Iraq is alarming. That is already half the death toll of Hiroshima. Apart from the practical arguments, the principled ones stand and will always stand. Have we not learnt any lessons from the history of sweeping alarming numbers of deaths under the carpet? This is not something about which there can be any political discretion 60 years after Auschwitz."
The Foreign Office said yesterday it believed the figures from the Iraqi ministry of health were the most reliable because they were based on head counts not extrapolation.