Poor planning, air strikes by coalition forces and a "climate of violence" have led to more than 100,000 extra deaths in Iraq, scientists claim.
Iraqis are now 58 times more likely to die a violent death, Lancet
A study published by the Lancet says the risk of death by violence for civilians in Iraq is now 58 times higher than before the US-led invasion.
Unofficial estimates of civilian deaths had varied from 10,000 to over 37,000.
The Lancet admits the research is based on a small sample - under 1,000 homes - but says the findings are "convincing".
Responding to the Lancet article, a Pentagon spokesman defended coalition action in Iraq.
"This conflict has been prosecuted in the most precise fashion of any conflict in the history of modern warfare", he said.
UK foreign secretary Jack Straw said his government would examine the findings "with very great care".
But he told BBC's Today that another independent estimate of civilian deaths was around 15,000.
The Iraq Body Count, a respected database run by a group of academics and peace activists, has put the number of reported civilian deaths at between 14,000-16,000.
The Lancet published research by scientists from the Johns Hopkins Bloomberg School of Public Health in the US city of Baltimore.
They gathered data on births and deaths since January 2002 from 33 clusters of 30 households each across Iraq.
They found the relative risk, the risk of deaths from any cause, was two-and-a-half times higher for Iraqi civilians after the 2003 invasion than in the preceding 15 months.
That figure drops to one-and-a-half times higher if data from Falluja - the scene of repeated heavy fighting - is excluded.
Before the invasion, most people died as a result of heart attack, stroke and chronic illness, the report says, whereas after the invasion, "violence was the primary cause of death".
Violent deaths were mainly attributed to coalition forces - and most individuals reportedly killed were women and children.
Dr Les Roberts, who led the study, said: "Making conservative assumptions we think that about 100,000 excess deaths, or more, have happened since the 2003 invasion of Iraq.
"Violence accounted for most of the excess deaths and air strikes from coalition forces accounted for most of the violent deaths."
He said his team's work proved it was possible to compile data on public health "even during periods of extreme violence".
The sample included randomly selected households in Baghdad, Basra, Arbil, Najaf and Karbala, as well as Falluja.
Lancet editor Richard Horton said: "With the admitted benefit of hindsight and from a purely public health perspective, it is clear that whatever planning did take place was grievously in error."
Civilian toll estimates at 10/04
Iraq Body Count: 14-16,000
Brookings Inst: 10-27,000
UK foreign secretary: >10,000
People's Kifah >37,000
Mr Horton concluded: "For the sake of a country in crisis and for a people under daily threat of violence, the evidence we publish today must change heads as well as pierce hearts."
No official estimate
There is no official estimate of the number of Iraqi civilians who have died since the outbreak of the war in Iraq.
Human rights groups say the occupying powers have failed in their duty to catalogue the deaths, giving the impression that ordinary Iraqis' lives are worth less than those of their soldiers for whom detailed statistics are available.
However, the Pentagon spokesman said "there is no accurate way to validate the estimates of civilian casualties by this or any other organisation".
He added: The loss of any innocent lives is a tragedy, something Iraqi security forces and the Multi-National Force painstakingly work to avoid.
"Former regime elements and insurgents have made it a practice of using civilians as human shields, operating and conducting attacks against coalition forces from within areas inhabited by civilians."