David Miliband repeatedly brandished his new Labour credentials yesterday in the strongest indication yet that he could stand for leader if Gordon Brown leaves office.

The Foreign Secretary staked his claim as leader of the next generation of Blairites by vowing a commitment to wealth creation and aspiration.

His comments follow Labour’s defeat in the Crewe & Nantwich by-election, in which the party’s campaign focused on attacking “Tory toffs” and the Conservative candidate’s personal wealth, leading David Cameron to pronounce new Labour dead.

His interview on Sky television, in which he mentioned “new Labour” eight times, saw Mr Miliband give one of his strongest defences of the Blairite project, in terms rarely seen since Mr Brown took over.

Mr Miliband also pointedly refused to rule out standing for the job if there was a vacancy, replying that he was “not in the market for any job other than the one I have got at the moment”. Earlier, Alan Johnson, the Health Secretary, categorically ruled out suggestions that he would stand if there was an opening.

The Prime Minister’s next big hurdle comes a week today in front of the Parliamentary Labour Party, where a strong performance is crucial for his political survival. Next month he will face the toughest vote of his premiership on the issue of a 42-day period of detention without charge. He will also have to weather controversy over MPs’ pay.

However, many believe that his real test will come before the conference season and the European and local council elections next year. Few Labour MPs would stand for a repeat of the heavy local election defeats suffered on May 1.

Mr Brown is also facing a testing time with the unions, who are negotiating the successor to the Warwick agreement, a wishlist of policies that Labour promises the unions it will enact.

There are signs that the unions are preparing to flex their muscles, with Tony Dubbins, Labour’s linkman to the unions, telling The Times that Mr Brown will lose the organisational and financial support of the unions without a radical policy shift to the left. Speaking in a personal capacity, the head of the Trade Union & Labour Party Liaison Organisation (Tulo) — which acts as a link between the 16 Labour-supporting trade unions and the party — called for a windfall tax on oil giants, higher business taxes and forcing employers to allow flexible time off for parents.

Yesterday five members of the Cabinet, including the Foreign Secretary, toured television studios to insist that there was no appetite to depose the Prime Minister despite last week’s disastrous showing in Crewe. Privately, a majority of ministers fear that Labour cannot turn around its current problems with Mr Brown in charge. Members of the Cabinet believe they cannot stand by and do nothing if the party seems to be heading towards certain defeat at the next election.

In public, ministers were defiant, with Mr Miliband rallying to the defence of the Prime Minister, saying he was “elected as the right man for the job last year and he is the right man for the job this year”.

Yesterday Mr Miliband denied he was involved in any “plot” to oust Mr Brown. But in a stark warning to colleagues, he said that the party must have the determination to keep fighting and show voters that the Government was on their side.

“David Cameron actually, inadvertently, put his finger on it. He’s not seeking just to bury Gordon Brown, he’s seeking to bury an entire political project, the new Labour political project that has turned Labour from being a natural party of opposition to being a forceful party of government, and I think we have got to show that we are up for that fight.”

Although many MPs and members of the Government are deeply unhappy with Mr Brown, they fear that a new leader would have little time to turn around the party’s fortunes and might have to hold a general election on assuming the post. This was challenged by a leading Blairite member of the Government yesterday, who told The Times that any new leader could have six or eight months in office before holding an election, so long as they set out a clear public timetable when they took over.

In an interview on the BBC, Mr Johnson insisted that the issue of the leadership was settled and there was no appetite for changing the leader. “Gordon Brown was the towering figure a year ago and is the towering figure in the party now,” he said.

However, he did appear to concede that Mr Brown was not at ease with the showbusiness of politics, saying: “He’s not lights, camera, action.”

This came as the trade unions are preparing to reassert their authority, and will shortly present a report to ministers showing how union supporters were turning increasingly to opposition parties.

Mr Dubbins said: “Why is it a Labour Government has alienated a lot of support from working people? I think certainly in core Labour areas there isn’t a shadow of doubt the BNP has made inroads and that’s an extremely dangerous development.

“If the Government continues down the track of being too pro-business and not friendly enough to workers, and does not readopt the kind of values we want to see, I think that there’s a big danger you will lose support from the unions. You won’t win general elections without trade union activists.

“I’m putting down a marker that the underlying issues, to do with values, are of fundamental importance.”

This is a particularly worrying warning for Mr Brown because the union contribution to party coffers grew to 88 per cent of the total in the first quarter of 2008, compared with 52 per cent at the start of last year.

Union members are already turning to the Liberal Democrats and the Conservatives for support, and resolutions have been tabled for the conferences of Unison, the biggest public sector union, the GMB general union and the Communication Workers Union, calling for a split with Labour.

Asked whether union members were turning to opposition parties, Mr Dubbins said: “Judging from what I’ve seen, it’s a mixture of the Lib Dems and Tories. We are doing studies ourselves internally, and will release this in a few weeks.

“Very few trade unionists are going to the BNP, but you get more of that in working-class constituencies where there are problems of housing, agency workers’ rights and migrant workers undercutting wage rates, creating xenophobia.”

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