A total of 24,865 civilians were killed in the two-year period until March this year, and women and children accounted for almost 20 percent of all civilian deaths, said the report.
The report, entitled A Dossier on Civilian Casualties in Iraq, 2003-2005, was launched by the Iraq Body Count in association with the Oxford Research Group after a comprehensive analysis of over 10,000 media reports published between March 2003 and March 2005.
Almost half of the deaths occurred in the capital Baghdad, the report said.
30 percent of civilian deaths was recorded during the invasion phase before May 1, 2003 when US President George W. Bush announced the end of "major combat operations" in Iraq.
After the invasion, the number of civilians killed was almost twice as high in year two, 11,351, as in year one, 6,215.
According to the report, most of the deaths were caused by foreign troops rather than by insurgents within the country. US-led forces killed 37 percent of civilian victims while 9 percent were killed by anti-occupation forces or insurgents.
More civilians, or 45 percent, were killed by explosives, including air attack, than by any other type of device, with air strikes causing most, or 65 percent, of the explosive deaths, the report said.
Children were disproportionately affected by all explosive devices but most severely by air strikes and unexploded ordnance, the report added.
"The ever-mounting Iraqi death toll is the forgotten cost of the decision to go to war in Iraq," Professor John Sloboda from Britain's Keele University, one of the report's authors, said on Tuesday at the launch of the report in London.
"On average, 34 ordinary Iraqis have met violent deaths every day since the invasion of March 2003. Our data show that no sector of the Iraqi society has escaped. We sincerely hope that this research will help inform decision-makers around the world about the real needs of the Iraqi people as they struggle to rebuild their country," said Sloboda.
"It remains a matter of the gravest concern that, nearly two and half years on, neither the US nor the UK government have begun to systematically measure the impact of their actions in terms of human lives destroyed," he said.