It was only weeks ago that Peter Mandelson was, in the words of a senior Tory source, “dripping pure poison” about Gordon Brown in his ears. These were not just passing remarks but a well-constructed critique of the prime minister’s failings born of intimate knowledge. The government’s vulnerability to the charge of overloading the country with debt was also discussed, I can reveal.

This last observation was made to a figure well able to understand its potency — and deploy the advice as the Tories formulated their autumn attack on Labour’s record. Debt subsequently became one of the key themes of the Conservative party conference in Birmingham last week.

I have absolutely no doubt that Mandelson has now put his anger behind him and is prepared to serve loyally his old comrade and former enemy, the prime minister. His formidable political talents can never be underestimated, and his enthusiasm for his new role is palpable. But past lapses such as the one above explain some of the misgivings in Brown’s entourage about the prodigal son’s return.

Princess Diana famously confessed the cause of her unhappiness was that “there were three of us in this marriage, so it was a bit crowded”. This description could just as well be applied to the friendship between the three architects of new Labour from the moment that Tony Blair became the favourite to lead his party and Mandelson chose to help him.

Blair has left Labour’s royal family, so there is room in the marriage again. “You cannot discount the emotional aspect,” says a friend of Mandelson about the reconciliation with Brown. You also can’t discount the measure of the prime minister’s desperation to survive and go on to win the next general election. The most famous vendetta in contemporary politics has been put aside for now.

“There is no friendship at the top,” Lloyd George once said. It is just as true to say there is no room for pure hatred at the top. Brown’s unusually bold stroke comes under the latter category.

Although the evidence I produce above illustrates the risk that the prime minister is running, what other choice did he have? The polls show that, despite Brown’s successful conference speech and more confident handling of the financial crisis, Labour is still lagging 12-14 points behind the Tories in the polls. On the economy, voters trust David Cameron and the shadow chancellor, George Osborne, more than Brown and his chancellor, Alistair Darling. A Labour leadership challenge was only in abeyance. It was time to throw the dice. Brown’s reward is that he is at last making the political weather.

Mandelson is famously accident-prone, twice losing office in ridiculous circumstances. He may not be third time lucky, given his terrible relations with the media. Even so, the prime minister has shown he can be generous to an enemy, the mark of a strong man.

Brown is now anxious to prove that his National Economic Council will be the blueprint for reform around the world, citing the political philosopher Francis Fukuyama on the need to regain trust in financial institutions. He tells me ill feeling between Mandelson and his own faithful ally Ed Balls, the schools secretary, is “nonsense”. Maybe, but a bit of trust-building in that quarter wouldn’t go amiss.

Brown’s jibe at the Labour party conference in Manchester that “this is no time for a novice” is now being followed up by a strategic challenge to Cameron and Osborne. The prime minister has created a team of “serious people for serious times” to reinforce his argument that experience trumps youth.

For, whatever his faults, Mandelson’s role in the world trade round as European commissioner and his earlier cabinet role in the same sector allow him to hit the ground running. His presence in what friends call an inner “war cabinet” to deal with the economic crisis counters Cameron’s cheeky offer to come to the aid of the government in its time of trial. What need of a coalition of enemies when Brown can work with Mandelson? How flattering to be part of a notional war cabinet, too, when his revered grand-father, Herbert Morrison, was home secretary in Churchill’s real wartime coalition.

The Tories think it’s all nonsense. A war cabinet consists of four to five people, not 19, as in the new economic line-up. Mandelson’s appointment is, they believe, a misguided return to former glories. Last week at the Conservative conference Cameron countered Brown’s taunting with a witty line of argument borrowed from McCain’s Democrat opponent, Barack Obama — “we have experienced his experience”.

Cameron was greeted by former critics in the press as a prime minister in waiting, while Osborne again impressed. But the Tories do have cause to fear Mandelson’s contribution to the direction of this government. His role in shaping Labour’s general election campaign may be pivotal, too, if Douglas Alexander, the prime minister’s disgruntled ally, is shunted aside.

The “third time lucky” cabinet minister knows that Brown’s government is weak through lack of focus. Unlike the Brownites, he has underestimated neither the appeal of Cameron and Osborne to middle Britain nor the attractions of change. If he can push Brown towards the “hard centre” of British politics, ie, public service reform and an enabling state, he could steal some of the Conservatives’ thunder. The Tories have hitherto found coherence by adopting a radical agenda on schools and welfare. Mandelson is a true “heir to Blair” so he will fight any opposition attempt to usurp this title.

The stakes are again high. If Mandelson doesn’t succeed in influencing the agenda — outside this immediate crisis and his department — his return will be seen as a public relations gimmick with a limited shelf life. For it is balanced by the reappointment of Nick Brown, a prime ministerial crony whom many Blairites fear and loathe, as chief whip. Damian McBride, the prime minister’s press adviser, who is also blamed for briefing against ministers — McPoison, as he is familiarly known — is kicked upstairs. (Compared with some of his predecessors, I found his advice, against the prevailing view, to be honest and unbitter.)

Mandelson’s other bête noire, Brown’s former Treasury press spokesman Charlie Whelan, is still highly visible. In his latest incarnation, as political director for the giant trade union Unite, Whelan has been behind four-letter-word briefings, doubting the intellect and parentage of Brown’s young rival David Miliband, just as he once fulminated against poor Peter. This has an importance beyond the clash of petty personalities, because the unions are intensely suspicious of Mandelson. They always regarded him, in the terminology of Maoists, as a bit of a “capitalist roader”.

Throughout the year, Brown has had discussions with politicians of the stature of Charles Clarke, the former home secretary, Alan Milburn, the former health secretary, and other leading dissidents. These talks, while friendly, got nowhere. In a New Statesman interview it was reported that ministers around Miliband had been urging Brown to bring Mandelson back. Certainly his appointment staves off a threat from disgruntled Blairites in cabinet following the possible loss of the Glenrothes by-election on November 6. Blairites out of office will watch this space carefully to see whether their new champion prevails.

Last week I advised that the prime minister needed a heavyweight to balance his lopsided authority during the economic crisis. In Labour governments Roy Jenkins once propped up Harold Wilson, and Denis Healey supported James Callaghan in similar circumstances to today. Callaghan even considered bringing back his predecessor, Wilson, as chancellor, according to the newly published diaries of his senior policy adviser Bernard Donoughue.

In a manner neither I nor anyone else expected, Brown has listened and acted. True, the prime minister has kept Alistair Darling as chancellor, the usual post for a balancer. The financial mistakes of the past year have been made in No 10 not 11, so perhaps this was prudent: old friends may not always be forgiving of new injuries. Mandelson is a big enough figure to provide an alternative heavyweight.

The test will be whether No 10 gives its star signing the requisite authority, and whether Mandelson can avoid the political epitaph that “he was his own worst enemy”. Well, at least he now knows it is not Gordon Brown.

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