Jan-Peter Balkenende lent political support to the Iraq war
An inquiry into the Netherlands' support for the invasion of Iraq says it was not justified by UN resolutions.
The Dutch Committee of Inquiry on Iraq said UN Security Council resolutions did not "constitute a mandate for... intervention in 2003".
The inquiry was launched after foreign ministry memos were leaked that cast doubt on the legal basis for the war.
The Netherlands gave political support to the 2003 invasion of Iraq, but had no military role.
The report demolishes the Dutch case for supporting the invasion, says the BBC's Europe correspondent Jonny Dymond.
It could also be taken to reinforce the international case against the Iraq war, he says.
The report accuses ministers of a selective use of intelligence reports, and says Prime Minister Jan-Peter Balkenende "gave little or no leadership to debates over the Iraq question", which was steered by the foreign minister at the time, Jaap de Hoop Scheffer.
Mr Balkenende formally thanked the committee for its report, but said he needed time to study it before responding.
Mr Balkenende decided to join the "coalition of the willing" assembled by US President George W Bush because, he said, Saddam Hussein had consistently flouted UN resolutions and possessed weapons of mass destruction.
The Dutch parliament opposed the decision to back the invasion.
Committee chairman, Willibrord Davids, said the Netherlands' loyalty to its alliance with the US and UK had taken precedence over the need to ensure the legality of the invasion.
The committee said there had been no UN mandate for the attack, putting the decision to join at odds with international law.
It said "the wording of [UN Security Council] Resolution 1441 cannot reasonably be interpreted as authorising individual member states to use military force".
Iraq's alleged breach of Resolution 1441, which gave Iraq "a final opportunity to comply with its disarmament obligations", was used by the coalition, and the Netherlands, to justify its invasion.
However, a memo from the time by Dutch foreign ministry lawyers, subsequently leaked, suggested the war was in fact illegal under international law.
The inquiry said there was no evidence to support rumours that the Dutch military took part in the invasion.
While the government has always said its military was not involved, questions had been raised because Dutch troops were taking part in exercises nearby at the time.
Dutch forces became part of the stabilisation force that was deployed to Iraq in the years after the war.
The Dutch parliament is likely now to consider whether the prime minister misled parliament, and whether to launch a formal parliamentary inquiry.