Despite his assessment, Kelly concluded that military action seemed the only way to "conclusively disarm" Iraq, the UK's Observer newspaper reported on Sunday.
His stance is not hawkish enough to comfort the government of Prime Minister Tony Blair as Kelly's widow prepares to give evidence to the inquiry on Monday.
"The US, and whoever willingly assists it," wrote Kelly, "should ensure that the force, strength and strategy used is appropriate to the modest threat that Iraq now poses."
More than 6000 civilians have been killed and 20,000 injured by the occupying US-led coalition, and Iraq has been plunged into anarchy, guerrilla warfare and economic meltdown.
Given the continued failure to unearth any weapons of mass destruction it seems that Kelly's doctrine of appropriate force has been exceeded.
Kelly, widely acknowledged as Britain's leading expert on banned Iraqi weapons, killed himself last month after becoming embroiled in a major political row over whether Blair exaggerated the case for war against Saddam Hussein.
The newspaper printed a previously unpublished article it said Kelly had written a few weeks before the war under condition of anonymity and which it said would be sent to the Hutton inquiry.
Kellyhas admitted that the figures for the weapons stickpiles that he gives are based on pre-1991 estimates, based in turn upon untrustworthy declarations by Saddam himself.
"Although the current threat posed by the Iraqi military is modest ... it has never given up its intent to develop and stockpile such weapons for both military and terrorist use"
Dr David Kelly
He also wrote that the evidence for post-1991 weapons development was based only upon "indications".
"Iraq has recovered chemical reactors destroyed prior to 1998 for allegedly civilian activity, built biological fermenters and agent dryers, and created transportable production units for biological and chemical agents and the filling of weapons."
This does not constitute a "current and serious" threat as Blair claimed, and certainly does not point to the much-questioned UK claim that Iraq had chemical and biological weapons that could be ready for firing within 45 minutes of an order.
Kelly's concern was more with Iraq's intentions, and the article could be seen as suport for Bush's policy of taking preemptive strikes against belligerent countries before they are capable of retaliation.
"Although the current threat posed by the Iraqi military is modest ... it has never given up its intent to develop and stockpile such weapons for both military and terrorist use," he wrote.
Kelly said any co-operation with weapons inspectors had been superficial.
"After 12 unsuccessful years of UN supervision of disarmament, military force regrettably appears to be the only way of finally and conclusively disarming Iraq," Kelly said.
"War may now be inevitable," he concluded.
The Hutton inquiry is examining the circumstances leading to the death of civil servant Kelly, who was found with a slit wrist last month just days after appearing before a parliamentary committee. It is due to report in October.