Poor planning, air strikes by coalition forces and a "climate of violence" have led to more than 100,000 extra deaths in Iraq, scientists say.
A study published by the Lancet claims the risk of death by violence for civilians in Iraq is now 58 times higher than before the US-led invasion.
It condemns the coalition's planning on public health as "grievously in error".
The Lancet admits the research is based on a small sample - under 1,000 homes - but says the findings are "convincing".
Scientists from the Johns Hopkins Bloomberg School of Public Health in the US city of Baltimore gathered data on births and deaths since January 2002 from 33 clusters of 30 households each across Iraq.
They found the relative risk of death was one-and-a-half times higher for Iraqi civilians after the 2003 invasion than in the preceding 15 months.
That figure jumps to two-and-a-half times higher if data from Falluja - the scene of repeated heavy fighting - is included.
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."
He went on: "Democratic imperialism has led to more deaths not fewer. This political and military failure continues to cause scores of casualties among non-combatants."
He urges the coalition forces to rethink their strategy to "prevent further unnecessary human casualties".
"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," he said.