Analysis By Nick Assinder Political Correspondent, BBC News website

Labour MPs used to have many answers to the question: "What is John Prescott for?"

Tony Blair continues to offer his support

He was a vital link between Tony Blair and grassroots party members and the unions, bringing some working class credibility and perspective to New Labour.

He was a great "fixer" who could hammer out deals between warring ministers and occasionally knock heads together. Most recently he has been called on to use those skills to oversee the eventual transfer of power from Tony Blair to Gordon Brown.

And since 1997 he has held some pretty big government portfolios - everything from transport to local government, not to mention running the country when Tony Blair is away.

Now, however, an increasing number of Labour MPs are struggling to answer the question "what is Mr Prescott for?" and some are even publicly stating he has no further role.

His credibility has taken a serious knock over recent revelations about his affair with his secretary and his apparent attachment to his grace and favour mansion - not helped by pictures of him playing the "toffs' game" of croquet.

Salary and perks

It appears many grassroots party members and, in particular, women Labour MPs, have been dismayed by his behaviour, which has undermined his authority.

And, while he may still be seen as the man best placed to ease the transition of power from Blair to Brown, his ability to command respect across the party, the Commons and the wider Labour movement has also been affected.

Affair with his secretary hit Prescott's standing

And, lastly, he no longer has a department to run. He has kept his titles of deputy prime minister and deputy party leader, plus salary and perks, but faces questions over what he actually does to earn them.

There is also the prospect that each time he stands up in the Commons he will be met by a blizzard of ridicule from the opposition benches - something the toughest minister can find difficult to cope with.

Yet, despite all this, Mr Prescott appears determined to resist all demands for him to resign, claiming he is subject to a media feeding frenzy. And the prime minister and senior colleagues have rallied to his support.

His friend and biographer, journalist Colin Brown, has revealed the prime minister has assured Mr Prescott they will not leave office separately but "go together".

Clean sweep

That, of course, goes to the heart of it, but possibly in an unintended way

Mr Blair undoubtedly means he and his deputy will retire together, as planned, and at a time of his own choosing.

But the danger is that if Mr Prescott is forced out early, the prime minister might have to follow in quick succession.

For a start, it may not be feasible or even possible under party rules for Mr Prescott to be removed from the cabinet but remain as deputy party leader.

And if he loses that party job, an election for a successor would follow which would inevitably see some Labour backbenchers claiming it was the ideal time for a clean sweep at the top, with an election for leader as well.

The likely time for that would be at this year's party conference in September.

There are already a number of would-be deputies quietly campaigning to replace Mr Prescott and that is unlikely to stop no matter how defiant he remains.

And, while he will be able to brush away thinly-veiled attacks from his old enemy Peter Mandelson, he will find it more difficult to ignore comments from more significant backbenchers.

For the time being, however, and with the prime minister's continued backing, Mr Prescott appears secure. Certainly he will continue to run the country while Mr Blair is on holiday this week.

Just how intense the pressure on him will become once the Commons recess ends next week and MPs start looking forward to what promises to be a dramatic autumn conference, remains to be seen.