By Nyta Mann BBC News Online political correspondent

The prime ministerial succession, and whether Gordon Brown will get it, is an underlying seam that has run through New Labour since before it took office in 1997.

What's more, having just seen off a spate of headlines on the state of the Blair-Brown political marriage ahead of last Tuesday's pre-Budget report, no one at the top of government is looking for an early return to the subject.

Only Gordon Brown and Tony Blair know if a deal exists between them

But the chancellor's own comments on Breakfast with Frost, when asked if Tony Blair had agreed to stand down in order to allow Mr Brown to take over as prime minister, not only left Sir David "gobsmacked" on his own sofa but gave the story fresh life.

In fairness to Mr Brown, if he had flatly denied that any such understanding existed between himself and Mr Blair, that too would certainly have made headlines - as happened when the prime minister did just that ahead of the June general election.

It is a question for competing memoirs to resolve in the future

But Mr Brown could have airily dismissed the invitation to dignify idle tittle tattle by engaging in it. Or he could have said that when he and Mr Blair meet, they don't waste their time on fripperies like the latest bar-chatter at Westminster.

Both are excuses he has been happy to use before when a question isn't to his liking.

In short, there are better non-denial denials he could have issued than the "what Tony Blair and I have talked about is something very private" answer he gave, and which leaves the door open to renewed speculation.

'Deal' pre-dates government

The supposed "deal" between the two men dates back to 1994 when Mr Brown held back from running for the Labour leadership following John Smith's death.

No-one except the two men who supposedly struck the agreement at a private meeting know if it actually exists.

Gordon Brown waits in the wings of the Frost on Sunday set

Even then, it is entirely possible, perhaps even probable, given Mr Blair's remarkable negotiating skill of conveying to those he wishes to win over that he has told them what they want to hear, that Mr Brown left that meeting believing the younger man would not be serving more than two full terms.

It is a question for competing memoirs to resolve in the future.

What we have known since June, though, is that any understanding, expectation or mere hope that the prime minister will call it a day before the next election must have been drastically reduced by the result of the last.

The 2001 general election returned a parliamentary majority in effect no different from the whacking great landslide of 1997. The same analysts who concluded that the scale of that first victory assured Labour of winning the next one now say the same of the election of 2005/6.

This commanding position, observers conclude, will have at the very least shunted back by four or five years the notional timetable of any Blair-Brown deal.

Place in history books

In the meantime moreover, and according to those in Mr Blair's circle, the prime minister may have more clearly heard the call of history.

He has already guaranteed his entry into the record books by having won, in 1997, the largest majority of any post-war prime minister. He is also the first Labour leader to have won his party two successive, secure terms in office.

To out-stay Margaret Thatcher, Tony Blair must go on to the end of 2008

But the historic landmarks still stretch before him. Winning the next election would make him the first Labour prime minister to win three terms. That takes us to 2005/6.

To displace Margaret Thatcher as the longest serving post-war prime minister, Mr Blair must out-stay her 11 years and 209 days, which takes us to the end of 2008.

These and other milestones are still to come.

Mr Brown has already held the title of Next Prime Minister for at least four years. It is in many ways a dangerous position to be in, certainly for any length of time.

The heir apparent rarely gets to claim the crown - as Willie Whitelaw in 1975, Michael Heseltine in 1990 and Michael Portillo this summer can confirm.

Rivals hate you and nurse thwarting your victory as their chief aim. The natural ebb and flow of politics take their toll, pushing the claims of others to the fore and your own further back.

And all the while, the progress of your ascent to the top is the subject of febrile speculation, which has its own debilitating effect on your chances of actually getting there.

Widely considered the greatest intellect in the cabinet and famed for his highly acute political antennae, it cannot have escaped Mr Brown's notice that the longer Mr Blair goes on, the more likely the crown is to slip from the grasp of the man set to inherit it.