Parliament can now, if it wishes, provide the missing legitimacy for war in Iraq. And it can do so only in a particular manner, which I will later describe. As matters stand, legitimacy has been drained away by two developments: the lack of full support from public opinion, and concerns about the legality of the enterprise under international law. This is why thousands of posters carried in the great peace march bore the slogan: "Not In My Name".

Parliament can now, if it wishes, provide the missing legitimacy for war in Iraq. And it can do so only in a particular manner, which I will later describe. As matters stand, legitimacy has been drained away by two developments: the lack of full support from public opinion, and concerns about the legality of the enterprise under international law. This is why thousands of posters carried in the great peace march bore the slogan: "Not In My Name".

To see why legitimacy matters, put yourself in the boots of our troops gathering on the Iraq border. Their spirits, their comfort of mind, their willingness to face death and not turn away depend upon their bond with their fellow citizens at home.

Are they fighting for something they can believe in? We provide that. Are we willing them to victory or are we not? If public opinion is doubtful and the legality of military action in question, it is hard for soldiers to summon up their last reserves of daring and will power. I would go further and say that it is dangerous, and the result may be tragedy if we ask our troops to fight without our wholehearted assent.

This uncertain situation stands in sharp contrast to earlier times. Whenever we have sent such a substantial proportion of our armed forces into battle, the reason has commanded near universal acceptance, if not enthusiasm. The troops who went off to the Continent in 1914 were cheered at railway stations as they departed for the Channel ports. In 1939 there was no joie de vivre but instead a general acceptance that a grim duty had to be done. The Falklands War was a popular cause.

In this regard, the position of the American troops massing alongside our forces in the desert is quite different. They can tell themselves that they have come to avenge the attack on the twin towers in New York. Some 3,000 people were killed on American soil. They can make the argument that war in Iraq is the first stage in rendering their country safe again. Indeed American public opinion as measured in the polls is much more supportive. No member of the US government is threatening to resign. For American troops, waging war in Iraq can be seen as their patriotic duty.

For British troops the situation is much more ambiguous. How can this be resolved? By Parliament. The most important fact about Britain's constitutional arrangements is that all power, actual and latent, is contained in a single chamber furnished with green benches, the House of Commons. The two Houses of Parliament will debate the rights and wrongs of an attack on Iraq before the action begins, which means perhaps as soon as tomorrow. The Government is to be applauded for making this opportunity available, if indeed events do turn out in this way.

However, one break with precedent would have to be made if the debate is to release its full potential. There must be no instructions from party leaders as to how MPs are to vote. There must be no activity by the party whips. The vote must be free. Free? The political establishment will be aghast at this proposal. Their system depends upon party discipline. The more important the subject, they think, the more necessary it is for the Government to assert its will. Except that whipped votes no longer carry full conviction outside the Palace of Westminster.

In the same way, to take a different example, the manner in which the US tried to bully or bribe the uncommitted members of the UN Security Council to support a second resolution would have had the effect of weakening its force even if it had obtained majority backing.

Imagine the two possible outcomes under my proposal. In case one, an unwhipped House of Commons gives its support to the Prime Minister's policy. There would no doubt have been Labour dissent on quite a large scale. Most of the Conservative Party would have backed Mr Blair. The Liberal Democrats would have voted against the motion. But a majority would have been obtained and with it full, unarguable legitimacy.

This would be parliamentary democracy in its purest form – an informed electorate represented by members acting in what they conceive to be the public interest. In these circumstances, war would be truly "in our names". I think case one would be far and away the most likely outcome.

But case two? There is no majority for war. The Prime Minister must resign. British troops are stood down. The Americans go it alone. Meanwhile a new Labour government is formed.

Yes, a dreadful political disaster, but nonetheless legitimacy would have been maintained. Parliamentary democracy would have been immeasurably strengthened. The nation would review its foreign alliances and in the end, I believe, emerge all the stronger.