The 2003 US-led invasion of Iraq lacked legitimacy under international law, an independent commission probing Dutch political support for the still controversial war said Tuesday.

"There was insufficient legitimacy under international law for a military invasion of Iraq" for which the Netherlands gave political but no military support, commission chairman Willibrord Davids told journalists in The Hague.

The commission's report said the Dutch government decision "was based mainly on international political considerations."

And it said the Netherlands had wrongly interpreted UN Security Council resolution 1441, which gave Iraq a final chance to disarm, as authorising individual member states to use military force against that country.

The 551-page report was presented to Prime Minister Jan Peter Balkenende, also head of government in 2003, who welcomed a finding that the Netherlands never provided any military support for the invasion.

The opposition, meanwhile, has demanded a parliamentary inquiry.

The Hague has repeatedly justified its political backing for the invasion with then-Iraqi leader Saddam Hussein's repeated refusal to respect UN Security Council resolutions.

The United States and Britain, which led the action, cited Saddam's alleged possession of weapons of mass destruction. None were ever found.

The commission said the Dutch government's position was against then public opinion and with its own policies, which opposed enforced regime change.

"The Dutch government lent its political support to a war whose purpose was not consistent with Dutch government policy. It may therefore be said that the Dutch stance was to some extent disingenuous."

The US benefited from the political backing "since it increased support for the invasion at the global level," said the report.

The commission, which started its work in March last year, was set up by the government amid growing pressure from political parties and the public for a probe into claims that crucial data had been withheld from Dutch decision-makers who opted to support the invasion -- backed by all but two political parties in the Dutch parliament at the time.

The Netherlands sent about 1,100 troops to Iraq in July 2003 to take part in a post-invasion, UN-mandated Iraqi stabilisation force. They left Iraq in 2005.

The probe found that Dutch policy on the issue had been defined by the foreign ministry under then minister Jaap de Hoop Scheffer, who later became NATO secretary-general.

"The Prime Minister took little or no lead in debates on the Iraq question," it said.

But Balkenende said the commission had found no proof of a link between the Dutch political support and De Hoop Scheffer's later appointment and had found "no evidence" to support rumours that the Netherlands had made a clandestine military contribution.

But he made no reference to findings that the government ignored the "reserved" and "nuanced" information held by domestic intelligence sources on Iraq's alleged weapons of mass destruction, and had "adopted those facts that fit the pre-determined political position".

Last month, a former UN weapons inspector said former US president George W. Bush and British Prime Minister Tony Blair shared a conviction that Saddam was a threat and causing them to allegedly mislead the public.

An official inquiry has started in Britain, with Blair set to testify in the coming weeks on the intelligence used to make a case for war. His ex-chief spin doctor Alastair Campbell gave evidence Tuesday.