Behind the curtains there lies a guillotine

If Richard Nixon had been a British Prime Minister and something like the Watergate burglary had happened here - at Dolphin Square, perhaps - he would have come down to the House within a day or so and made a speech addressed chiefly to his backbenchers. We can all write the script: "Powerful evidence ... intelligence sources ... national security at stake ... still at stake ... trust me ... honest sort of guy..."

Mr Tony Blair would win a comfortable majority on a personal vote of confidence, with a few abstentions on his own side, and 40 or so voting against, variously described in the public prints as "the awkward squad" or "the usual suspects". The papers would headline their reports Triumph For PM or words to that effect. Such a course was not available to the US President over the episode in question. This leads me to conclude that, in some respects, America is a more punctilious country than we are.

This does not mean that Prime Ministers are always safe in their jobs: far from it. It is one of the errors of the theory of presidential government to suppose that they are. H H Asquith, David Lloyd George, Neville Chamberlain, Anthony Eden and Margaret Thatcher were all pushed out while they were in office. It is arguable that Winston Churchill should be added to the list as well. He did not want to make way for Eden in 1955 (with some justification, it should be added, in view of the way things turned out) and he would have hung on if he had been given any encouragement at all by his colleagues to stay.

There are, it may be noted, no Labour Prime Ministers on the list. Harold Wilson's resignation in 1976 was as big a shock to most of his colleagues as it was to everybody else, even though it had been planned by Wilson for several years. Labour does not like getting rid of its Prime Ministers, no doubt because there have been so few of them.

Consequently the party lacks practice, historically speaking. The party comrades and the brothers from the branches (by now historical phrases both) were devoutly grateful that they had a Labour Prime Minister at all. Indeed, the old party constitution - so full, so detailed, so determined to cover every contingency that it would have kept a Chancery Silk happily occupied for weeks - made no provision of any kind for removing a Labour Prime Minister.

Oddly enough, the new constitution does have such provisions. I write "oddly" because the story goes, and by and large it is perfectly true, that since 1994 our beloved leader has gradually taken more and more power to himself and to the Central Committee. But in one respect he has rendered himself more vulnerable, at any rate in theory, whether knowingly or not it is difficult to say. The new constitution says that it is indeed possible to remove a Labour Prime Minister.

The process requires, to begin with, a simple card vote (no fancy majority being needed) at the party conference, calling for an election to be held. The election then takes place by means of the new, improved electoral college, with a one-third share each for Labour MPs, constituency parties and trade unions. It is, by the way, one of the public-relations triumphs of Mr Blair's regime that even otherwise knowledgeable political reporters have been persuaded that New Labour has adopted a comprehensive system of one person, one vote.

So far, so simple: or so one might have thought. But no. It rarely is. The prospective new Prime Minister must be a Labour MP. That, admittedly, is simple enough. But the level of support which he or she is required to drum up to stand at all is either 20 per cent of the parliamentary party or 12.5 per cent. The higher figure is needed when the candidature is a challenge; the lower figure when it is to fill a vacancy. Which would it be if the conference decided - I am not saying, mind you, it would do anything of the kind - that it wanted to dispose of Mr Blair?

I am now offering an opinion free, gratis and for nothing which, if it had come from the Chancery Silk referred to earlier, would cost the equivalent of a month's holiday in the South of France, perhaps longer. It is that it all depends on Mr Blair. He could either accept the conference's decision and announce his imminent departure or proclaim his intention of contesting the election against all comers. In the former circumstances, all likely lads (and lasses) would have to possess a 12.5 per cent level of support; in the latter, the proportion would go up to 20 per cent.

If Mr Blair took the first course, of deciding to resign, there would be no need for him to resign immediately as Prime Minister. So there would be no interregnum by Mr John Prescott, who as Deputy Prime Minister would have clear precedence over Mr Gordon Brown. This was what happened in 1976, when Wilson carried on for a few weeks until James Callaghan had been elected leader of the party and had succeeded him as Prime Minister. It was likewise what happened in 1990, when Mrs Thatcher stayed for Mr John Major to be safely elected before herself bidding a lachrymose farewell to No 10.

Both these transfers of prime ministerial power, the first voluntary, the second involuntary, were carried out without the slightest constitutional difficulty, embarrassment or dispute. Various "constitutional experts" of the time - some of whom, such as Lord St John of Fawsley, keep up a practice to this day - had maintained confidently that troubles of all kinds would unfailingly follow. In particular, the Queen's prerogative to choose her Prime Minister would be compromised. But all the evidence is that she and those famous advisers of hers are only too glad to lose this prerogative and welcome the system of prime ministerial election which both parties have now embraced.

Though the old Labour constitution did not provide for the removal of a Prime Minister, the parliamentary party nevertheless supported election after death or resignation but had never had occasion to use it. Election would have been by the older (and better) system where MPs were the sole voters. The Conservatives came round to election not long after the royal appointment of Alec Douglas-Home in 1963, which did the Monarchy even greater damage than that of Harold Macmillan six years earlier. I do not expect Mr Blair to be the first example of a displaced Labour Prime Minister. But when he comes down to the House on Tuesday to try to justify himself yet again, it will do him no end of good to know that behind the curtains there is now a guillotine.