In a controversial comment piece, understood to have been penned last weekend while staying in his constituency home in South Shields, he says: "The odds are against us, no question."

In a wide-ranging list of criticisms of the party, he said that NHS reform wasn't started quickly enough, plans for Iraq were substandard, and that more power should have been devolved from Whitehall and Westminster.

The former environment minister added that Britain needs a 'clearer drive towards becoming a low-carbon, energy-efficient economy'.

He made no reference to Gordon Brown anywhere in the 1,000 word article.

Rather than backing the Prime Minister, he has instead fired a warning shot to Conservative leader David Cameron, saying: "I disagreed with Margaret Thatcher, but at least it was clear what she stood for.

"The problem with David Cameron is the reverse. He may be likable and sometimes hard to disagree with, but he is empty."

David Miliband is currently 3/1 to become the next Prime Minister, down from 7/1 in April. He is now second favourite to Conservative leader David Cameron.

Political analyst and writer James Forsyth said David Miliband was trying to emulate the success of presidential candidate Barrack Obama.

He said: "The Foreign Secretary clearly intends to portray himself as the youthful candidate who can reinvigorate the project, the so-called Milibama strategy."

Mr Miliband is still pushing ahead with a series of 'profile-raising' regional visits, officially referred to as an "outreach tour," around the time of Labour Conference in September, when speculation suggests a challenge to Mr Brown is most likely.

Mr Miliband, who is set to holiday in Menorca this summer, ended by urging colleagues to "enjoy a break, and then find the confidence to make our case afresh".

Former chairman of the South Shields Labour Party, Coun Geraldine White, said: "David is a man of honour, and that is a quality which is highly prized in politics.

"He doesn't promise anything unless he can deliver.

"I endorsed him when he came to South Shields as MP in 2001, because he
was open, honest and frank.

"On a personal level, he's very funny, he was a great sense of humour, and he adores his family."

Former Mayor of South Tyneside, Cathy Brown, said today: "David Miliband is a highly respected, principled politician.

"If he were to succeed in a leadership challenge, then local grassroots members would back him wholeheartedly, along with the Labour members of local government here in South Tyneside."

A senior South Shields Labour party source said: "David's chances of becoming the next Prime Minister are still slim, Gordon Brown has battled for years to get to where he is and he won't give it up without a fight.

"But someone has had to pull the trigger to start a realistic leadership battle, and David may have done just that this morning."

Yesterday South Shields-based political author Jack Grassby said: "A year is not long enough to judge Gordon Brown."