Will he go this year, next year, or wait until the end of his third term? Tony Blair has left us with a riddle worthy of a Dan Brown novel. Political Editor Chris Lloyd attempts to crack it.

NO matter who calls for it, what wings of the party they are on, or how many of them there are, it is ludicrous to expect Tony Blair to publish a timetable laying out when he will leave office.

A timetable will only be a gift for the Conservatives and a sure way to plunge Labour into a full-blown civil war.

The only way out for Mr Blair, whenever the time comes, is for a short, sharp goodbye. To pick up the phone and say: "It's all yours, Gordon. I leave tonight."

Just as Harold Wilson did in March 1976.

Many of Mr Blair's current problems are that he felt forced to publish a vague timetable of departure before the last election. He said he would serve a third term, but no more. This enabled people to work out that he'd go in late 2008 - leaving his successor enough time to get his feet under the table for a General Election which has to be held by May 2010.

Almost immediately, the jockeying began. The Prime Minister no longer looked immortal, his word was no longer final - and so people began casting around for the words of his successor, the man who would have the ultimate say. Then they began currying favour with the man who they would like to have the ultimate say. And - a year after an historic, large third victory - before you could say "Nick Robinson", the party appears on the brink of civil war.

Imagine, then, if Mr Blair set a definite timetable. For example, he said he was leaving next May.

Come Christmas, the Labour Party would be ungovernable and the country would be drifting.

The leadership candidates would have their own feuding camps (there will, incidentally, be candidates beyond Mr Brown: Charles Clarke and John Reid appear ideally placed at the moment, while Darlington MP Alan Milburn notably refused to rule out the possibility of standing against Mr Brown); there would be resignations from the Government as people disassociated themselves from the out-going leader; Mr Blair would only make decisions that Mr Brown agreed with.

There would be paralysis in Government and great crisis in Labour. And the resurgent Conservatives would be having a long party. One Blairite minister has suggested that, as an amusing stunt, they'd place a large countdown clock outside Labour headquarters, ticking away the months until the nation said "bye-bye, Blair".

At midnight on the day of his departing, they'd throw a party. They would rejoice, while Labour tore itself apart. Who would look like the party of Government then?

The biggest loser of such a scenario would be Mr Brown. Mr Blair already has his achievements, he already has his legacy - the man who made Labour electable, who won three huge victories and became longest-serving leader in the party's history (yes, the leader who took the country into an unpopular war on a flimsy pretence with no exit strategy, but history can be selective).

Mr Brown has yet to begin to make his history. To do that, he has to have, as he says, a "stable and orderly transition". However desperate he is to get into No 10 as quickly as possible, he cannot allow the party to disintegrate in the process. If it does, he'll be a footnote in history: the man who squandered three elections and let the Tories back in.

Mr Brown is a shrewd man. He sees this. So why is he making so much trouble?

This is the fascinating aspect to the "civil war" stories. Is Mr Brown doing the fighting? In fact, is anyone publicly waging this war? Because, despite the rumours, no backbenchers have yet admitted to physically signing a letter calling for Mr Blair to go.

There have, it is true, been new additions to the ranks of those calling for Mr Blair to consider his position - the avuncular Nick Raynsford as big a surprise addition as Ashok Kumar who set hares running in this newspaper a month ago with his call for "a smooth and rapid transition".

But most of those trying to force Mr Blair out are those who wanted him out on May 2, 1997 - the Lynne Joneses and Glenda Jacksons, genuine left-wingers.

They, in all conscience, want to renew the party as Old Labour.

But is Mr Brown the man to do this? Having served as Chancellor throughout Mr Blair's reign, he is wedded to Mr Blair's policy. He has been with him through thick and thin, through Kosovo and Iraq.

A change of leader may bring a change of emphasis, but it will certainly not bring a change from New to Old. Indeed, this changed leader may well have room only to change the emphasis and not the policy - Mr Brown has planned to slow the growth in public expenditure to meet his golden rules, so he won't have the finance to reverse many of Mr Blair's policies even if he wanted to.

And Mr Brown won't want to ride into power on a wave of the left's making, for fear that he would find himself washed up with some shipmates not of his own choosing.

It is also rather presumptuous to think that Mr Brown would have saved Labour from embarrassment in the local elections. Labour did OK in its northern heartlands for a midterm Government - even winning a couple of seats in Hartlepool. It was in London and the south-east that it lost ground to the Conservatives.

Would Mr Brown - the tax-raising Chancellor with a dour, Scottish streak of old socialist Labour about him - have appealed more to this "middle England" than Mr Blair? Or were the local election results more about a combination of Labour's ineptitude and, just as important, the resurgence of the Conservatives under David Cameron?

Of course, Mr Blair could make all of this speculation redundant by picking up the phone and saying: "It's all yours, Gordon. I leave tonight."

Yet Mr Blair was the one who promised the country before he won his third large victory that he would "serve a full third term". Could he go back so blatantly on that and depart with any honour?

Working out what he meant is a riddle worthy of the da Vinci code. It'd be silly to expect him to serve right up to the end of a third term. So he was always going to serve 'almost a third term'.

When he was out in Australia earlier this year, it was briefed that he planned to go on until 2008. In a speech in Trimdon less than two months ago, he said he wanted to "lay important foundations as our thinking starts to turn to our fourth term programme" - a sentence which hints at a 2008 or even 2009 departure.

Yesterday, though, he appeared to be moving those goalposts. No longer was he talking about a fourth term or even serving a full third term; instead he was talking in terms of honouring "the commitment to a stable and orderly transition - and I repeat I will honour it, with the time plainly needed for my successor to establish himself".

Da Vinci-esque, you can read into that what you want. It does sound a little sooner than what he said in Trimdon, but it doesn't sound as if his departure is imminent. It doesn't sound as if it will be this year.

In an adult world, Mr Brown would have some real insight into unravelling this code, but the ending of the Blair/Brown Government now seems wrapped up in the running sore that has disfigured so much of its rule: the uncomfortable relationship between its two leading protagonists.

But the moment Mr Blair publishes that insight as some form of timetable, what is left of his authority will ebb away forever. He may be a lame duck now; a public timetable would really cook his goose - and that is what those calling for it want to do.