The Government's case for going to war in Iraq has been torn apart by the publication of previously suppressed evidence that Tony Blair lied over Saddam Hussein's weapons of mass destruction.

A devastating attack on Mr Blair's justification for military action by Carne Ross, Britain's key negotiator at the UN, has been kept under wraps until now because he was threatened with being charged with breaching the Official Secrets Act.

In the testimony revealed today Mr Ross, 40, who helped negotiate several UN security resolutions on Iraq, makes it clear that Mr Blair must have known Saddam Hussein possessed no weapons of mass destruction. He said that during his posting to the UN, "at no time did HMG [Her Majesty's Government] assess that Iraq's WMD (or any other capability) posed a threat to the UK or its interests."

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Mr Ross revealed it was a commonly held view among British officials dealing with Iraq that any threat by Saddam Hussein had been "effectively contained".

He also reveals that British officials warned US diplomats that bringing down the Iraqi dictator would lead to the chaos the world has since witnessed. "I remember on several occasions the UK team stating this view in terms during our discussions with the US (who agreed)," he said.

"At the same time, we would frequently argue when the US raised the subject, that 'regime change' was inadvisable, primarily on the grounds that Iraq would collapse into chaos."

He claims "inertia" in the Foreign Office and the "inattention of key ministers" combined to stop the UK carrying out any co-ordinated and sustained attempt to address sanction-busting by Iraq, an approach which could have provided an alternative to war.

Mr Ross delivered the evidence to the Butler inquiry which investigated intelligence blunders in the run-up to the conflict.

The Foreign Office had attempted to prevent the evidence being made public, but it has now been published by the Commons Select Committee on Foreign Affairs after MPs sought assurances from the Foreign Office that it would not breach the Official Secrets Act.

It shows Mr Ross told the inquiry, chaired by Lord Butler, "there was no intelligence evidence of significant holdings of CW [chemical warfare], BW [biological warfare] or nuclear material" held by the Iraqi dictator before the invasion. "There was, moreover, no intelligence or assessment during my time in the job that Iraq had any intention to launch an attack against its neighbours or the UK or the US," he added.

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Mr Ross's evidence directly challenges the assertions by the Prime Minster that the war was legally justified because Saddam possessed WMDs which could be "activated" within 45 minutes and posed a threat to British interests. These claims were also made in two dossiers, subsequently discredited, in spite of the advice by Mr Ross.

His hitherto secret evidence threatens to reopen the row over the legality of the conflict, under which Mr Blair has sought to draw a line as the internecine bloodshed in Iraq has worsened.

Mr Ross says he questioned colleagues at the Foreign Office and the Ministry of Defence working on Iraq and none said that any new evidence had emerged to change their assessment.

"What had changed was the Government's determination to present available evidence in a different light," he added.

Mr Ross said in late 2002 that he "discussed this at some length with David Kelly", the weapons expert who a year later committed suicide when he was named as the source of a BBC report saying Downing Street had "sexed up" the WMD claims in a dossier. The Butler inquiry cleared Mr Blair and Downing Street of "sexing up" the dossier, but the publication of the Carne Ross evidence will cast fresh doubts on its findings.

Mr Ross, 40, was a highly rated diplomat but he resigned because of his misgivings about the legality of the war. He still fears the threat of action under the Official Secrets Act.

"Mr Ross hasn't had any approach to tell him that he is still not liable to be prosecuted," said one ally. But he has told friends that he is "glad it is out in the open" and he told MPs it had been "on my conscience for years".

One member of the Foreign Affairs committee said: "There was blood on the carpet over this. I think it's pretty clear the Foreign Office used the Official Secrets Act to suppress this evidence, by hanging it like a Sword of Damacles over Mr Ross, but we have called their bluff."

Yesterday, Jack Straw, the Leader of the Commons who was Foreign Secretary during the war - Mr Ross's boss - announced the Commons will have a debate on the possible change of strategy heralded by the Iraqi Study Group report in the new year.