In 2003, on the day of a Labour cabinet reshuffle in which he was tipped for a significant promotion by Tony Blair, the radical, reforming health secretary Alan Milburn stunned Westminster by announcing that he was quitting frontline politics altogether to spend more time with his family.

That expression had been so over-used, even abused, by others who were quitting for less than honourable reasons, that the sound of MPs spluttering in disbelief could be heard echoing around Westminster.

One pundit immediately declared Milburn was standing down "to spend more time with his ambition", another claimed it was "a blatant leadership bid".

And his move was certainly seen by many as part of a long, calculated campaign that would ultimately see him bidding for the leadership as the Blairite torch bearer when the prime minister finally resigned.

Whatever his reason though, it didn't work out the way the sceptics had predicted and Milburn became one of the many would-be leaders of the time who were bulldozed aside by the Gordon Brown bandwagon as it single-mindedly powered him into the job unopposed (probably the biggest mistake of Brown's political life, as it turned out).

Despite, or more likely because of his impeccable modernising credentials and record as NHS reformer, had he thrown his hat in to the ring he would have faced an uphill struggle. Not through lack of talent but because Brown and his formidable, relentless campaign seemed unstoppable and, in any case, Labour was well andf truly out of love with Blair and the Blairites.

Milburn's hopes had also suffered a setback after he was brought back into the front line in 2004 to head Labour's general election campaign. It proved a relative disaster and he was quickly moved aside to make way for Gordon Brown who took to the role with his characteristic vigour.

For many in his own party, who always suspected the tag "moderniser" really meant "Tory", his next job was probably less of a surprise, but that didn't stop them whispering "traitor" behind his back.

Tory collaborator

After the coalition was formed in 2010, Milburn became the coalition's social mobility tsar. At the time, and on occasion since, there were rumours that he had been offered a Peerage and full cabinet post under David Cameron, but they were never substantiated.

None the less, the appointment brought fierce criticism from former deputy prime minister John Prescott, amongst others, who attacked him and other ex-ministers Frank Field and John Hutton who had also agreed to advise the coalition.

Prescott tweeted: "So after Field and Hutton, Milburn becomes the third collaborator. They collaborated to get Brown out. Now collaborating to keep Cameron in" and former health secretary Andy Burnham said his decision was a "kick in the teeth" for Labour.

The "joke" went around that it was now clear Milburn had stood down from the cabinet to "spend more time with the Tories".

For his part, Milburn insisted he would be independent of the government and act as a watchdog and not a lapdog.

However, there were also claims that he was fed up that reforms he had proposed when he was Prime Minister Brown's Social Mobility Commissioner had not been implemented.

He has since proved true to his promise and has produced reports for the coalition suggesting just the sort of radical reforms to end the downward spiral in social mobility that marked him out as a moderniser during his years in Blair's cabinet.

His analysis of the problem has been welcomed and accepted by the prime minister and Labour and chimes with very similar sentiments offered by former Tory prime minister Sir John Major.

So just about everybody agrees with his description of the problem and accept his warnings about the bleak future awaiting many youngsters from ordinary backgrounds unless immediate, long-term action is taken.

But he is again facing the real prospect that, just as under Labour, his suggested reforms are just too radical and difficult and could simply be kicked into the long grass, or cherry picked.

He has pledged to keep up the pressure and "hold the government's feet to the fire" on the issue and his job certainly gives him a platform from which to do that.

Foundation Trusts

Milburn's political career started in Newcastle where, after leaving Lancaster university, he helped run a small radical bookshop called "Days of Hope" which was known locally as "Haze of Dope", for some reason (cough).

He worked for the Trade Union Studies Information Unit in the mid-1980s and was elected as Chairman of Newcastle-upon-Tyne Central Constituency Labour Party. In 1990 he worked for North Tyneside Borough Council and became an official for the North East Region of the Manufacturing, Science and Finance Union .

He won the seat of Darlington in the 1992 general election and was a close friend and ally of Tony Blair who made him a health minister in 1997 from where he piloted the controversial Private Finance Initiative.

When Peter Mandelson was forced to resign in 1998, he became chief secretary to the Treasury and then, in 1999 was promoted to Health Secretary where he continued his programme of modernisation, including the creation of Foundation Trusts.

He was always seen as a leading torch bearer for the Blairite, modernising wing of the Labour government and for some years was touted, along with David Miliband, as one of the generation who could take over the project when Blair finally stood down.

Despite no longer being in government, he remains as committed as ever to his modernising agenda and has sought ways of expressing it, which have included working with the coalition but also offering advice to both the government and Labour.

He has major reservations about the government's health reforms, however, having described them as a privatisation and "a car crash, a tsunami, a megastorm".

So it is no surprise that, when then health secretary Andrew Lansley asked him to chair the new clinical commissioning board, he turned the offer down. Then, in 2012 it was reported that a Downing Street adviser had launched an extraordinary attack on Lansley for the way he had handled the hugely-controversial NHS reforms.

He was reported telling a journalist: "Andrew Lansley should be taken out and shot. He's messed up both the communication and the substance of the policy." And he suggested Milburn should be put in the Lords so he could be made health secretary to replace Lansley.

Neither Cameron or Milburn were attracted by the suggestion and to this day, Milburn remains on the outside.

However, few believe he plans to fade away and, if he remains true to his words and his recent performance is anything to go by, he will continue to press his reforms on both Cameron and Labour.

What many now wonder, however, is whether his persistence will see him become an irritation to Cameron by constantly providing a very public reminder of how little things have changed in relation to social mobility.

If that happens, he might suffer the same fate as previous radical thinkers, and suddenly find himself once again spending more time with his family.