Miliband emerges as the young pretender

The 43-year-old Foreign Secretary answered a stream of questions about whether the Prime Minister should remain as leader with the repeated formula "Gordon will lead us forward".

His words left open the possibility of Mr Brown being toppled at any time - and were taken by the Prime Minister's allies as deliberately hostile.

There was little doubt at Westminster that Labour was on the brink of a civil war, possibly breaking out as early as the autumn.

Mr Miliband was repeatedly asked throughout the day to endorse Mr Brown.

"Gordon will lead us forward and the rest of us have a contribution to make," he told a packed press conference when a visit by the Italian foreign-minister was turned into a media circus.

His warmest words for Mr Brown were: "Can Gordon lead us into the next election and win? Yes. I'm absolutely confident about that."

That was in reply to the specific question put by the Evening Standard on whether he thought Mr Brown was the only person to lead Labour into the election.

Mr Miliband was asked to rule himself out as a future Labour leadership candidate. He replied: "How many times do I have to say that this is about arguments, about issues, about a united Labour response? On Friday, I said that Labour had to pull together to address the big issues facing the country."

Pressed further on whether Labour would be "mad" to change its leader before the next election, Mr Miliband again ducked the question, saying: "As a loyal member of the Labour party, I would say no, the party never does mad."

Today's extraordinary drama was provoked by a 900-word article written by Mr Miliband and setting out his vision for fighting the next election on a radical policy reform agenda.

His article appeared to set out a leadership stall and failed to defend the beleaguered Prime Minister.

Allies of Mr Brown were furious. The events climaxed at a press conference where Italian minister Franco Frattini looked bemused by the attempts of the Press to pin Mr Miliband into endorsing Mr Brown to stay - and the minister's verbal gymnastics as he avoided giving such a commitment.

Asked if he was seeking the leadership, he replied: "I'm not campaigning for anything other than a successful Labour Government."

"I think that my article today was a challenge to David Cameron and not a challenge to Gordon Brown," Mr Miliband added.

His motive was frustration at " fatalism" in the party ranks, he said.

The Foreign Secretary's actions surprised allies of Mr Brown who saw them as a calculated attempt to undermine the Premier.

One accused him of being "disloyal" and "self-serving" and of lacking " judgment and maturity".

Two former ministers from Tony Blair's time weighed in behind Mr Miliband.

Peter Mandelson said Labour was "in some flux" and former Europe minister Denis MacShane said "Labour MPs should follow Miliband's leadership".

Mr Brown, weakened by by-election defeats and dreadful opinion poll results, could not risk sacking the minister.

One ally of the PM said: "I think MPs will be appalled. David Miliband has shown himself to be not only disloyal but also self-serving. People at least thought he was a serious figure and a grown-up politician but by allowing his head to be turned by this leadership nonsense, he has revealed a surprising lack of judgment and maturity."

Mr Miliband is due to go on holiday later this week and his move was seen as an attempt to steal a march on possible rivals such as Jack Straw and Harriet Harman.

Mr Miliband could have been in no doubt that any failure to endorse the Premier fully today would be seen as another hostile act.

But when approached by the Standard this morning he merely said: "I'm making a contribution to the debate about how Labour addresses the needs of the country and responds to the Glasgow East by-election. That's something we should all be doing."

Challenged later by Sky News to back the Prime Minister, he said: "Gordon Brown is the leader of the Labour Party and he will lead us forward to address the big issues." MPs believe Mr Miliband may have written his Guardian piece because of concerns that Leftwingers have gained ground ahead of a possible leadership contest.

Labour backbencher Jeremy Corbyn dismissed the article as "shallow", adding: "It fails to address the concerns of core Labour voters, housing prices or pensions for example."

In his article, Mr Miliband described how he would rescue Labour. He said: "New Labour won three elections by offering real change, not just in policy but in the way we do politics. We must do so again."

Bookmakers slashed the odds on Mr Miliband being the next Labour leader and prime minister to 9/4 from 5/2.

Labour MP Ian Gibson said: "It does feel like a leadership challenge." Health Secretary Alan Johnson, tipped as a running mate, said he was "not talking about that, thank you very much".

Mr Miliband is said to be assembling a leadership campaign team.

Commons Leader Ms Harman again denied positioning herself for a leadership campaign, saying: "When a woman says no, she means no."