The man once regarded as the world's most powerful banker has bluntly declared that the Iraq war was 'largely' about oil.

Appointed by Ronald Reagan in 1987 and retired last year after serving four presidents, Alan Greenspan has been the leading Republican economist for a generation and his utterings instantly moved world markets.

In his long-awaited memoir - out tomorrow in the US - Greenspan, 81, who served as chairman of the US Federal Reserve for almost two decades, writes: 'I am saddened that it is politically inconvenient to acknowledge what everyone knows: the Iraq war is largely about oil.'

In The Age of Turbulence: Adventures in a New World, he is also crystal clear on his opinion of his last two bosses, harshly criticising George W Bush for 'abandoning fiscal constraint' and praising Bill Clinton's anti-deficit policies during the Nineties as 'an act of political courage'. He also speaks of Clinton's sharp and 'curious' mind, and 'old-fashioned' caution about the dangers of debt.

Greenspan's damning comments about the war come as a survey of Iraqis, which was released last week, claims that up to 1.2 million people may have died because of the conflict in Iraq - lending weight to a 2006 survey in the Lancet that reported similarly high levels.

More than one million deaths were already being suggested by anti-war campaigners, but such high counts have consistently been rejected by US and UK officials. The estimates, extrapolated from a sample of 1,461 adults around the country, were collected by a British polling agency, ORB, which asked a random selection of Iraqis how many people living in their household had died as a result of the violence rather than from natural causes.

Previous estimates gave a range between 390,000 and 940,000, the most prominent of which - collected by the Johns Hopkins Bloomberg School of Public Health and reported in the Lancet in October 2006 - suggested 654,965 deaths.

Although the household survey was carried out by a polling organisation, rather than researchers, it has again raised the spectre that the 2003 invasion has caused a far more substantial death toll than officially acknowledged.

The ORB survey follows an earlier report by the organisation which suggested that one in four Iraqi adults had lost a family member to violence. The latest survey suggests that in Baghdad that number is as high as one in two. If true, these latest figures would suggest the death toll in Iraq now exceeds that of the Rwandan genocide in which about 800,000 died.

The Lancet survey was criticised by some experts and by George Bush and British officials. In private, however, the Ministry of Defence's chief scientific adviser Sir Roy Anderson described it as 'close to best practice'.