David Miliband and Harriet Harman are preparing the ground for a leadership election, Times Online can reveal.

Many in the Labour party now believe that a revolt against Gordon Brown is highly likely, with the probability that it will take place at the start of September.

A number of ministers are considering standing down in early Autumn, possibly refusing to serve in a Brown reshuffle, it was reported by The Times today.

It is alleged that Ms Harman was spotted in the aftermath of Labour's defeat in the Glasgow East by-election telling aides "this is my moment". This comes despite public protestations of loyalty from Labour's deputy leader, who is "minding the shop" in Downing Street this week while Mr Brown is on holiday.

"I think most people have accepted that it is over and only a handful of people in Downing Street are trying to stop anything happening," said a senior government source.

Friends of James Purnell, the Work and Pensions Secretary, have confirmed that he has reached an agreement with David Miliband so that he would represent the Blairites in any contest. He is understood to have been quietly sounding out colleagues about a Miliband candidacy.

Senior party figures are urging Alan Johnson to agree to serve as Mr Miliband's deputy in a so-called "dream ticket".

The operation to canvass support for the Foreign Secretary includes senior Labour figures in the Lords, who have been contacting peers to prepare the ground for a bid. He is believed to be the best hope for the Blairite wing of the party, despite his failure to challenge Mr Brown for the leadership last year.

Meanwhile Ms Harman's private speech to the National Policy Forum in Warwick on Saturday night was seen as a bid for the leadership.

As well as making jokes at the expense of Jack Straw, another possible contender, she laid out the need to put equality and fairness at the centre of the Labour government - believed to be the centrepiece of any campaign.

Labour sources believe she has a serious chance of becoming leader if a contest is held. "The public do not have a say in any contest, just the Labour Party, and people easily forget she won the last time a vote was held. She has quietly been working on the unions, with her husband Jack Dromey, and is popular amongst the membership, as well as having the backing of a large number of MPs."

Ms Harman tonight hastily issued a statement denying she was gearing up for a challenge. I am not preparing the ground for a leadership election," she said.

“In respect of Labour’s defeat in the Glasgow East by-election, I did not tell aides - or any one else - that ’this is my moment’.

“I was bitterly disappointed by the Glasgow East by-election result in which I campaigned in support of Margaret Curran - a woman who I admire greatly.”

She added: “My ’public protestations of loyalty’ to Gordon Brown are no different to what I have expressed in private. I do not accept ’it is over’.”

There are fears in Labour circles that up to six candidates could stand - like the deputy leadership contest - because the signatures of only 45 MPs are needed to enter the race. However, one source believes that the race would fold down into a contest between Ms Harman and Mr Miliband.

Mr Brown's performance at the National Policy Forum on Friday lunchtime, where he talked at length about India and China and the significance of the iPod, made by US giant Apple, is being spoken about as the event that convinced some they had reached the point of no return.

The most significant outcome of the Forum - which was meant to agree an agenda for the next Labour manifesto - was an agreement with the unions to sit down again in the next 18 months. This is because union leaders think there is little point in negotiating with Mr Brown before the next election.

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