The Big Question: How does a political party oust a serving Prime Minister?

Are there any rules?

Anyone plotting to prise the Prime Minister out of Downing Street will no doubt have consulted the Labour Party Consolidated Rule Book. This says that there is an opening every summer to nominate someone else to be the party leader, during the run-up to the Labour Party annual conference. But anyone who wants to take on Tony Blair in a contested election has to collect the signatures of at least one in five Labour MPs. That is 71 MPs, in the current Parliament. And even if someone were to turn up at a party conference with the required number of nominations, the conference has to vote on whether they want an election or not. If they say no, Tony Blair can carry on unchallenged.

The final point that makes it all irrelevant is that nominations for this year have already closed, nearly three months ago. The only nominations submitted were for the old team of Tony Blair and John Prescott.

Is there any other method?

No. The next chance to try to force an election will not come around until the next annual conference in September 2007. Nobody expects Tony Blair still to be in office by then, and even if he hung on that long, and someone with enough support to attract 71 nominations from Labour MPs, such as Gordon Brown, decided to take him on, at that point even Tony Blair would surely realise that the game was up and would quit rather than put up a fight. So the formal procedure for removing Tony Blair is meaningless. Those who want him out will have to devise a scenario in which he feels he has no choice but to resign.

What happens if a prime minister refuses to resign?

That was a live question in the mid-1950s, when Prime Minister Winston Churchill was over 80 years old and exhibiting signs of deterioration, but could not bear the thought of retiring. This created a situation not unlike today's: his acknowledged successor had always been Anthony Eden, who was Foreign Secretary through the whole of Churchill's premiership (which was interrupted by five years of Labour government) and was chafing at the bit to take over. When Churchill at last gave up, in 1955, Eden's nerves were shattered, and his was the shortest and unhappiest premiership in post-war history. But Churchill's case was exceptional: his reputation made him untouchable and his great age meant that this colleagues knew he could not last much longer.

Another famous example of a prime minister being reluctant to leave 10 Downing Street when his time was over was Sir Henry Campbell-Bannerman, who led the Liberals to victory in the 1906 general election, when he was almost 70. His health broke down, he resigned from office, but still would not move out of 10 Downing Street, where he died three weeks later. That also was exceptional. When a prime minister's political authority has collapsed, by one means or another, his not-so-loyal colleagues will find a way to get him out.

How did it used to be done?

Go back far enough, and Prime Ministers quit when Parliament sacked them. Every government from 1837 to 1874 was brought down by a parliamentary vote. That happened again in 1886 and 1895, and, much more recently, in May 1979, when Jim Callaghan lost a vote of confidence in the Commons, by 311-310 and was forced to call a general election. Before that, the last Prime Minister forced out of office in that way was Ramsay MacDonald in 1924.

But by far the most important example of a prime minister being levered out of office by the Commons happened in a more roundabout method. On 8 May 1940, after the Germans had overrun Norway, the Commons held a technical vote on the motion that "this house do adjourn", in which 33 Conservatives and eight other pro-government MPs voted with the opposition and 60 abstained. This still gave the government a majority of 81 - which under any normal circumstances would be regarded as overwhelming - on a meaningless vote, but the Prime Minister, Neville Chamberlain, took the hint and resigned anyway, making way for Winston Churchill.

Can't Parliament vote a PM out?

Party loyalty holds MPs together in a way it never did in Victorian times, and the surest way to save a party leader who is in trouble is for other parties to join in. In 1993, John Major lost control of the Conservative Party, and lost an important vote in the House of Commons. He immediately tabled a motion of confidence in his own government, defying the Commons to sack him. But sacking him would have triggered a general election in which a lot of Conservative MPs would have lost their seats. So the whole Tory party rallied to the government. Similarly, if David Cameron tabled a vote of no confidence in the Labour government, every Labour MP would rally to Tony Blair.

Well, then, what method is left?

The short answer is "senior colleagues" - or as they were once known in parliamentary slang, the "men in grey suits". Who these men were depended on the circumstances of the day, but they usually included the Chief Whip and key members of the Cabinet. Their job was to go as deputation to Downing Street and solemnly inform the Prime Minister that he had lost the confidence of his colleagues. There was much talk about these shadowy gentlemen in the final days of Margaret Thatcher's premiership, until their failure to step forward caused them to be renamed the "men in brown trousers".

How did the Tories remove Mrs Thatcher?

The rules for making an open challenge to a prime minister were much simpler in Margaret Thatcher's day than at any time before or since. All a challenger needed was one proposer and a seconder, who did not even have to make their names public, and so it was possible, in 1989, for a no-hoper, "stalking horse" to run against her.

Her downfall came a year later when the Deputy Prime Minister, Sir Geoffrey Howe, resigned and hinted that somebody ought to have another go at unseating her, and Michael Heseltine took up the challenge. She defeated Heseltine in the initial ballot, but the killer blow came when she held one-to-one conversations with members of her Cabinet after that vote. With Kenneth Clarke leading the charge, 11 out of 15 told her it was time to go. That seems to be the best answer for those who want to know how to force Tony Blair to resign: get the Cabinet to do it.

Could the problem be solved by term limits on office?

A president of the United States can serve for only eight years - two four-year terms - or a bit more if he was previously a vice-president who assumed office on the death of the then president. Would it not be possible to limit the total number of years a prime minister could serve in office - either continuously or in separate periods - in the same way? In theory, of course, a new convention or fresh legislation could require it; in the same way that a parliament cannot by law - unless it writes a new law - prolong itself beyond five years.

However, this way of making sure a prime minister does not outstay his - or her - useful days would so violate the traditional dependency of a prime minister on the support of parliament that it would have a hard time garnering much support. It would also deny the country the choice of person they might want to continue leading them.