Alan Milburn has stepped down from the government to spend more time with his family.

The former health secretary told the prime minister that he was "finding it increasingly difficult to balance having a young family in the North East with the demands of being a Cabinet minister".

In a move that took Westminster by surprise, the exit of the Blairite minister from the Cabinet was announced on Thursday morning.

Ahead of a full government reshuffle announcement, Number 10's official spokesman said there were no other reasons for the Darlington MP's exit.

He urged reporters, now engaged in a flurry of speculation about his motives, to take the reason for his decision "at face value".

In his resignation letter to Tony Blair, Milburn said "it is not a political decision, for I support you totally in what you are trying to do".

"There will be implications and there will be the wildest of conspiracy theories about this," he told journalists later.

"You get one shot in life with kids. You get one chance to see them grow up. I have not been there and I want to be there."

The MP, who lives with his psychiatrist partner Ruth Briel and two sons aged six and 12, appeared to rule out a return to frontline politics.

"I really can't envisage that, I just can't envisage that at all partly because the kids are so young," he said.

He denied he had ever harboured the ambition to lead Labour, despite being tipped by some - reputedly including Cherie Blair - as a possible successor to the prime minister.

"You always talked about my leadership ambitions, I never said I wanted to be leader of the Labour Party or prime minister or whatever," he told Channel 4.

Others suggest Milburn was growing increasingly frustrated by the actions of the chancellor.

The senior two ministers clashed over the borrowing powers of foundation hospitals and relations were said to be bitter between them since.

The prime minister was informed of the decision on Monday and unusually no hint was given of the news in the fevered reshuffle speculation.

Milburn said in his letter that "this is the right time to go...the NHS is now moving forwards".

"The money is going in and the reforms that you and I have agreed are making an impact," he added.

Milburn had angered sections of the Labour Party with his controversial plans for foundation hospitals - which will operate with greater freedom from Whitehall control.

Backbenchers fear the proposals will lead to a two-tier health service and begin the privatisation of the NHS.

Milburn has been one of the government's key delivery ministers since he entered government as number two at the Department of Health in 1997.

He was promoted to the Cabinet as Treasury chief secretary the following year, becoming health secretary soon after in 1999.

Since his appointment he has also overseen the implementation of the 10 year NHS plan and secured record spending increases for the department.

Blair told Milburn that he was sorry to see him go but expressed gratitude for "the straightforward way in which you communicated your decision to leave".

"You have made a major contribution to the success of this government. The NHS has clearly turned a corner," he wrote.

Former education secretary Estelle Morris denied that there were deeper reasons behind Milburn's resignation

"I don't think Alan is a man who would behave in that way," she said. "It's all about trade offs and he has made a decision that that is what he wants."

BMA chief Ian Bogle said his decision would be "remembered as the architect of the NHS plan".

"He and I worked together with a common bond to improve the NHS for the benefit of patients," he added.

Gill Morgan of the NHS Confederation said she was sad to hear the news. "The health service will miss his powerful support and advocacy," she said.

"Alan was the central architect of the NHS Plan and has played a major role in helping the service turn the corner."