Mandelson remains resolute in his support of Gordon Brown

Lord Mandelson could not have been more measured or more certain as he made a direct plea to Labour MPs: "Don't please, through your actions, make it any worse for the party."

The biggest of the cabinet beasts to support Gordon Brown and urge caution among panicking MPs, the business secretary put the talk of a plot to unseat the prime minister down to the "grumbly mood" of some backbench colleagues.

Over the last 24 hours, Mandelson has been plotting himself – behind the scenes, and in front of TV cameras – to defuse the rebellion against Brown, a position barely conceivable even a year ago.

In the last few weeks, he has emerged as the figure at the heart of the operation to save the prime minister: this, from a man whose views of Brown have often not been printable in a family newspaper.

There has not been a whisper in private or public, not even a trademark quizzical raising of the eyebrow, to suggest that he has any doubt Brown must be kept in office. He has been resolute, urging MPs to hold their nerve.

Once seen as one of the most divisive figures in Labour, he has been turned arch-peacemaker, urging MPs to remember Brown's qualities, and giving testimony that his admiration for Brown, as prime minister, has grown over the months. As one minister close to Brown put it: "One of the great things about politics is that you learn to live with irony."

It underlines the extraordinary ­reconciliation since 1994, when Mandelson publicly chose Tony Blair rather than Brown to back for the leadership on the death of John Smith, opening up a rift with Brown which deepened over more than decade, and one which many thought could never heal.

Yet since his phenomenal return to government in October's game changing reshuffle, Mandelson's influence in the cabinet , Whitehall and in Number 10 has grown exponentially.

There is probably only one other figure, Ed Balls, the children's secretary and putative chancellor, who now can rival Mandelson for influence in Brown's ear.

His total loyalty leaves some other ministers privately exasperated, since they know that Brown would be fatally weakened if Mandelson only breathed one Iago-like hint that he should step aside for the good of the party.

Indeed his loyalty leaves some Blairites reaching for psychoanalysis to try to understand what is going on, including a posited need to assuage past guilt.

Brown, in turn, has supported Mandelson over his highly contested plans for the partial privatisation of Royal Mail, in the face of a huge backbench revolt.

After being ejected from Cabinet twice over personal errors, this crisis also gives Mandelson the opportunity to reinstate himself and prove he can be a steady hand in the midst of a perfect storm.

One Brownite strategist at the heart of No 10 praises him to the skies. "When the hand grenades are being thrown in and the explosions are going off, he always has had the calmest clear assessment of the situation. He can predict where an issue will be three or four days ahead.

"I think it is just that he has been there so many times before. There is an air of authority to him. And, to be honest, it was something that No 10 lacked". Asked why Mandelson is so intent on saving Brown, he said: "My view is that his assessment is that 'if you think it is bad now and getting rid of Gordon won't make it any worse, he says 'think again'."

Mandelson believes Brown has an intellectual grasp of the issues, a resilience and seriousness no other cabinet politician can match. And if he were removed, there is very little chance a successor could withstand the media pressure for an immediate election, something the party is not yet equipped to withstand.

But there have been reports of tensions between him and Balls as the courtiers jostling for Brown's ear; in particular, suggestions Mandelson was excluded from the Wednesday afternoon ''shoot the breeze'' forward looking sessions chaired jointly by Balls and the Cabinet Office minister, Liam Byrne.In reality Mandelson can attend what he wants, and is always present at the more ad-hoc long term strategic sessions at which he excels. Balls was suspicious at first of Mandelson, one Brownite says, but on the major economic issues there have not been disagreements.

A Blairite former cabinet minister thinks Mandelson sees his role in ensuring Brown gets what he wants; as such, he would not stand in the way of Balls being made chancellor.

The alliance between Mandelson and Brown has proved strong. There is, perhaps, only one issue that could break this – the Royal Mail. Mandelson insists it is an issue he inherited and not of his making, and trusts Brown will not back down even though he faces a rebellion .

Should the prime minister start to ­wobble, Mandleson would feel aggrieved. And that is not something that Brown needs right now.