It has been clear for the past year to many of us, not least to senior military figures, that the time had come for British forces to leave Iraq.

It has been equally clear that Tony Blair could not deliver this without losing face. He and George Bush were irrevocably committed to the mirage of creating a new liberal democratic order in the Middle East. For them, to get out now would have been an admission of failure.

The arrival of Gordon Brown at No 10 creates a valuable new situation. The web that ensnared Blair has left Brown unencumbered. Despite his support for the invasion, Brown has never been part of that neoconservative "democratisation movement", that pattern of distorted intelligence and ideological propaganda which has led to and continues to fuel the obscenity which is Iraq today.

As Brown takes over he has a narrow window of opportunity to use his unique authority to bring this nightmare to an end. He can demonstrate boldness and vision. He can jettison the rhetoric of bringing freedom and democracy to a grateful nation. He can admit the truth: that the British people don't want us to stay in Iraq a moment longer, nor do our soldiers fighting there and, most importantly, nor do the vast majority of the Iraqi people.

He can face the facts. The Shias, whom we originally went in to liberate, are the same Shias who are killing our soldiers. We are no longer keeping the peace; we are increasingly the target for its breach.

We are no longer saving Iraq from terrorism; indeed, we were complicit in the dismantling of Iraq's security apparatus, which created the vacuum which enticed al-Qaeda in. We are no longer delivering a democratic Iraq; we are, with the Americans, presiding over a sectarian government sliding into civil war. We are no longer the "welcome liberators", but rather the "evil occupiers" around whose bombed vehicles the locals dance in chilling celebration.

We have no place in Iraq any more. The only reason we are still there is an astonishing adherence to the agenda of a discredited US president. That surely is not worth dying for.

In pulling them out now, Brown would not be letting our troops down. They have been heroes. They have done everything that has been asked of them, and more. It is politicians - and I admit that originally I was one - who got it wrong. It is time we got it right by simply getting out. We don't need to set a timetable. We are within a few hours of the border by road, and even less by air. If we decide to get out, we can do so almost instantaneously.

There are those who continue to argue that such a withdrawal would be followed by internecine warfare. Conversely, it could actually serve to bring a degree of albeit uneasy stability to a situation where our continued presence is an aggravation and irritation rather than a palliative. It would certainly be in the interests of Iraq's immediate neighbours - Iran, Turkey, Syria and Saudi Arabia - to ensure that it did.

Gordon Brown has a short but golden chance to end Britain's pointless engagement in Iraq, but he will have to do so quickly and with determination. He should grasp that chance.

Michael Ancram MP was shadow foreign secretary (2001-2005)