The UK Government will "examine with very great care" claims 100,000 Iraqi civilians have died as a result of the US-led invasion, Jack Straw has said.
Iraqi people are now 58 times more likely to die a violent death
A study in the Lancet said the majority of the victims were women and children.
The UK foreign secretary told BBC's Today that another independent estimate of civilian deaths was around 15,000.
The Lancet admitted the research was based on a small sample - under 1,000 Iraqi households - but said the findings were "convincing".
The study by US and Iraq researchers was led by Johns Hopkins Bloomberg School of Public Health, Baltimore, US.
It said poor planning, air strikes by coalition forces and a "climate of violence" had led to the deaths in Iraq.
The risk of death from violence for civilians in Iraq was now 58 times higher than before the war, it said.
The overall risk of death was one-and-a-half times higher, it added.
Violence was now the "primary cause of death" in Iraq, and the study attributed most of these deaths to coalition forces.
The major causes of death before the war were heart attack, stroke and chronic illness.
Mr Straw told the Today programme: "Because it is in the Lancet, it is obviously something we have to look at in a very serious way.
"It is, however, an estimate that is based on very different methodology from standard methodology for assessing causalities, namely on the number of
people reported to have been killed at the time or around the time they were killed."
The researchers compared civilian mortality during the 14.6 months before the invasion with the 17.8 months after it.
They interviewed 988 Iraqi households from 33 randomly selected neighbourhoods.
In those households reporting violent deaths since January 2002, the date, cause and circumstances were recorded.
Lancet editor Richard Horton said: "Democratic imperialism has led to more deaths, not fewer. This political and military failure continues to cause scores of casualties among non-combatants."
He urged the coalition forces to rethink their strategy to "prevent further unnecessary human casualties".
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.
Unofficial estimates of the civilian toll have varied from 10,000 to more than 37,000.