The hunt has begun to find an “anti-Blairite” candidate to take on David Miliband as party figures declared that the Foreign Secretary would not be allowed to run unopposed.

Potential leadership candidates who were quietly sounding out Labour MPs and unions, in expectation of a September race, have been forced to ratchet up their scouting for support.

However, none apart from John McDonnell, the left-wing Labour MP who tried to challenge Gordon Brown last year, is willing to go public now.

One source on the left of the party said: “People had been expecting the starting gun to be fired for this contest in September. But Miliband started much earlier than anyone expected. They must have a plan – or else they would be crazy – so now we have to find our own candidate.”

Senior party figures still hope to persuade Jack Straw, who is on holiday in the US, to stand, despite him being painted as a “caretaker” leader – implying that Labour MPs accept they will lose the next election.

The position of Jon Cruddas, who ran for the deputy leadership, is being watched amid suggestions that he has been urged to team up with Harriet Harman. He could deliver many of the union votes, which represent a third of the electorate in a leadership ballot.

Yesterday Ms Harman, who would be a formidable candidate in any contest, issued an unequivocal denial on Woman’s Hour on BBC Radio 4 that she was plotting leadership moves.

“The men in the House of Commons press lobby . . . keep saying, ‘Are you preparing a leadership bid?’ and I keep saying, ‘No’. But I think it’s one of those occasions where they won’t take ‘no’ for an answer. And when a woman says ‘no’, she means ‘no’.”

Yet Ms Harman remains the only figure to have won a leadership contest under Labour’s complicated election rules. She saw off Alan Johnson last June with 50.43 per cent of Labour’s electoral college to become deputy leader, largely because of her popularity among rank-and-file members. She came second among MPs and MEPs – helped by early unofficial backing from Mr Brown – but only fifth out of six among the unions.

Since then, however, she has been working as Equality Minister to deliver the family-friendly, flexible-work-ing agenda that the unions promote.

Her husband, Jack Dromey, is deputy general secretary of the Transport and General Workers Union, now Unite, and Labour’s treasurer, having been reelected last week. A band of loyal MPs, including Joan Ruddock and Chris Bryant, also support her. They helped amass support for her in Parliament during last year’s contest.

The past few months have mostly advanced her cause rather than hindered it. Strong performances at Prime Minister’s Questions against William Hague surprised many even on her own side. Ms Harman is helped by Ayesha Hazarika, a former lobbyist and press aide to Patricia Hewitt.

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