The war in Iraq had "no basis in international law", a Dutch inquiry found today, in the first ever independent legal assessment of the decision to invade.

In a series of damning findings, a seven-member panel in the Netherlands concluded that the war, which was supported by the Dutch government following intelligence from Britain and the US, had not been justified in law.

"The Dutch government lent its political support to a war whose purpose was not consistent with Dutch government policy," the inquiry in the Hague concluded. "The military action had no sound mandate in international law."

In a further twist, it emerged that the UK government refused to disclose a key document requested by the Dutch panel.

The document – allegedly a letter from Tony Blair asking for the support of the Dutch prime minister, Jan Peter Balkenende – was handed over in a breach of diplomatic protocol and on the basis that it was for Balkenende's eyes only, an inquiry official told the Guardian.

"It was a surprise for our committee when we discovered information about this letter," said Rob Sebes, a spokesman for the Dutch inquiry. "It was not sent with a normal procedure between countries – instead it was a personal message from Tony Blair to our prime minister Jan Peter Balkanende, and had to be returned and not stored in our archives.

"We asked the British government to hand over the letter but they refused," Sebes said.

Details of the Dutch inquiry's findings and the refusal of the British government to disclose the letter are likely to increase international scrutiny on the Chilcot inquiry, as it emerged that the UK was instrumental in influencing the Dutch decision to back the war.

"In its depiction of Iraq's [weapons of mass destruction] programme, the [Dutch] government was to a considerable extent led by public and other information from the US and the UK," the Dutch report says.

"This report is an objective finding – it was not political, we searched for the truth," said Sebes. "We think that over 10 months the seven members of committee made a real effort to make a finding of high quality."

Philippe Sands QC, a professor of international law, who gave evidence to the Dutch inquiry, said: "There has been no other independent assessment on the legality of the war in Iraq and the findings of this inquiry are unambiguous. It concludes that the case argued by the Dutch and British governments, including the then attorney general, Lord Goldsmith, could not reasonably be argued.

"This is the authoritative view of seven commissioners including the former president of the Dutch supreme court, a former judge of the European court of justice, and two legal academics."

The findings have prompted controversy in the Netherlands, where Balkenende has so far resisted calls for a formal parliamentary inquiry into his government's decision to back the war.

However, the report is likely to influence analysis of the events in the UK, experts say, with senior judicial figures among those concerned that the Chilcot inquiry lacks the expertise on issues of legality in comparison to the Dutch panel.

"The findings of the Dutch inquiry that the war had no basis in international law are even more important for a domestic audience in Britain," said Sands. "I do not see how the five members of the Chilcot inquiry, none of whom is legally qualified, could possibly summon the means to reach an alternative conclusion."