This verdict on the illegal war in Iraq demolishes Mr Blair's last line of defence

During the crucial months of diplomatic and political argument that preceded the war in Iraq, the Prime Minister advanced three separate reasons for joining President George Bush's mission to remove Saddam Hussein. The first, and the one Mr Blair relied upon to sway public and parliamentary opinion, was the imminent danger presented by Iraq's weapons of mass destruction. This has now been thoroughly discredited by the facts on the ground.

The second reason, and the one that clinched the argument with many on the left, was the moral and humanitarian consideration: Saddam Hussein was so cruel a dictator, responsible for so many deaths, that he had to be ousted. That argument has been, at very least, dissipated by the failure of the occupiers to enforce elementary security after the first, combat, phase of the conflict was over and by the documented abuse of Iraqi prisoners committed by American soldiers at Abu Ghraib.

The third reason, and the one that underpinned the first two for the purpose of persuading British MPs to vote for military action, was the legal argument. And as the other justifications for war fell away, so the importance of the legal argument grew. According to the published summary of the Attorney General's advice to the Prime Minister, the use of force was justified by Iraq's repeated failure to comply with UN Security Council resolutions on eliminating its banned weapons. Iraq's defiance had placed it on the wrong side of international law. And if the UN was not prepared to authorise this necessary action, individual Security Council members could, and would. This argument remained standing, though not unchallenged, until yesterday's bombshell from the UN Secretary General.

After months of signalling his disapproval by his silence, Kofi Annan has said, clearly and simply, that in his view the war was illegal. The furthest he had gone before that was to venture the opinion, when war looked inevitable, that the legitimacy of any action without a new UN mandate would be "seriously impaired". His reply to a BBC interviewer yesterday that from his point of view and from the point of view of the UN Charter, the war was "illegal" is a categorical statement if every there was one.

What we have here is the head of the United Nations completely dissociating himself and the UN from the US and British military action. To suggest that this was not what the British government - or a US administration in full election mode, or any other "coalition" government, for that matter - wanted to hear would be an understatement. But it is in Britain that the potential damage is greatest, because it was here that the war was most contested.

The torrent of self-justification that poured forth from Downing Street and Whitehall from early yesterday told its own story. Ministers, starting with Patricia Hewitt, the loyal, Blairite Trade and Industry Secretary, regretted what they insisted was a "disagreement" between honourable people, describing the war as "not only lawful, but necessary". There was disagreement, they said, between international lawyers over the legality of the war. But this is hardly the point. We knew there was serious disagreement among lawyers because there were resignations. We suspected also that the Attorney General's advice might not have been quite as clear-cut as ministers would have had us believe. Is it really only precedent that has deterred the Government from publishing that advice in full to this day? Political expedience, after all, has a way of trumping precedent when needs must.

No; the lethal reality for the Government is that this was not just another highly qualified international lawyer expressing a contrary view. This was the well-regarded Secretary General of the United Nations - the head of the very organisation whose authority the US and British supposedly waged their war to uphold. And the fact is that there, in that one crystalline word "illegal", went Mr Blair'slast justification for the war - and with it the last shreds of his credibility.