Health Secretary Alan Milburn quit the cabinet today - as Prime Minister Tony Blair began his long-awaited reshuffle.

The Darlington MP, who today announced his shock resignation from the Government, is a staunch Blairite loyalist who had been tipped as a possible successor to the Prime Minister in 10 Downing Street.

He was entrusted by Tony Blair with the vital task of turning the extra money poured into the National Health Service by the Government into visible improvements in the service received by patients.

His vision of the 21st-century NHS, involving increased involvement for private contractors and greater freedoms for local managers, chimed very closely with the ideas of the Prime Minister himself.

Mr Milburn's commitment to NHS reforms including the private finance initiative and foundation hospitals made him a controversial figure on the left of the party, and he clashed over pay with consultants, who rejected new contracts he offered them.

He was one of the few ministers to emerge triumphant from a battle with Chancellor Gordon Brown, securing significant financial freedoms for NHS trusts which take on foundation status.

But there is little doubt that Mr Blair regarded him as the right man to force through changes regarded as key to Labour's chances of success in the next General Election, and his loss will be a bitter blow for the Prime Minister.

There was no suggestion that any personal or political rift with Mr Blair lay behind his decision to quit the Cabinet to spend more time with his family.

Mr Milburn said: ''For me it was a personal choice and a straightforward one between a career in politics and a life with my family.

''For most people, that choice is a pretty straightforward one.''

Reacting to the suggestion that political commentators would be looking for ulterior motives behind Mr Milburn's decision, the MP replied: ''I understand that and they are wrong. I understand that there will be motives suggested about this.

''There will be implications and there will be the wildest of conspiracy theories about this.''

Mr Milburn added: ''But it comes down to a simple thing - a personal choice.''

Discussing his future, Mr Milburn said: ''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.''

Describing the kind of life politicians lead, Mr Milburn added: ''I think it's a crazy way of life and a mad way of life. People have different ways of dealing with it - but this was an intensely personal choice.''

Few figures in modern politics have moved more totally from the far-left fringe of the Labour movement to the respectable Blairite wing of the party than Mr Milburn.

He transformed himself from a street agitator and noisy, unkempt salesman of Marxist literature into probably the most loyal and trusted of the Prime Minister's Cabinet colleagues.

The Milburn of today, in his impeccable, expensive and tailored suits, is unrecognisable from the Milburn of yesterday, who used to frequent Newcastle's political bookshop, Days of Hope (which bore the unflattering nickname 'Haze of Dope') and campaigned rowdily against the closure of local shipyards.

After his election as MP for Darlington in 1992, his political potential was quickly spotted at Westminster and he was marked out as a high-flyer virtually from the moment Mr Blair took over the Labour leadership in 1994.

He was born on January 27, 1958, and brought up by a single mother in the County Durham mining village of Tow Law which he has since described as a ''staunch Labour environment''.

His mother was a secretary in the local NHS and almost certainly had to bring him up on a tight budget.

Mr Milburn attended John Marlay school in Newcastle, and Stokesley comprehensive in Cleveland, before going on to Lancaster University for his BA in history. He did not complete a PhD at Newcastle University on 18th-century radicalism in the North East.

But even as a young man he was at the forefront of local political activity.

He ran the campaign to save Sunderland's shipyards and is a past president of the Union for Manufacturing, Science and Finance in the North East.

Before entering Parliament, Milburn worked as a business development officer for North Tyneside Council.

He quickly made his presence felt at Westminster, becoming a prolific questioner, particularly on NHS issues. Before Labour swept to power, Mr Milburn served as an Opposition front-bench spokesman first on health and then on treasury and economic affairs.

Once Mr Blair was installed in Number 10 in 1997, Mr Milburn was appointed a junior health minister in charge of promoting PFI projects to build new hospitals in controversial public-private partnerships.

He threw himself into the job with relish, and in doing so incurred the wrath of some health workers by his tough stance on tackling ineffective hospitals and their managers.

This, plus a plan to stage NHS pay rises, was to earn him a barracking by delegates at a Unison conference in July, 1998. In 1998, he entered the Cabinet as Chief Secretary to the Treasury, regarded as the most arduous of ministerial posts. A year later he succeeded Frank Dobson as Health Secretary, the post he seemed tailor-made for.

His political journey was echoed in his private life, as he separated from his first wife, trade unionist Mo O'Tool, who was known as 'Red Mo' for her fierce principles and fiery speeches.

He is now happily settled with psychiatrist partner Dr Ruth Briel, with whom he has two sons.

It is the difficulty of combining the ''24-hour job'' of Health Secretary with family life in the North East that appears to have put a end - perhaps temporarily - to his political ascent.