As James Purnell quits, last flickers of the Blair Labour flame die out

• James Purnell

Two months before the general election, the man seen by many as a key figure in the party's future yesterday declared that he simply no longer wanted to be an MP.

The move is a major blow to his close friend, Foreign Secretary David Miliband, in the event of a post-election leadership contest. The pair were seen as the central figures for carrying the Blairite flame but Mr Miliband is now looking increasingly isolated.

It also adds to the list of MPs associated with the Blair project who are now going or already gone. They include John Reid, Alan Milburn, Patricia Hewitt and Stephen Byers.

While Mr Purnell has argued that the Blairite tag is now largely irrelevant, his departure will be seen as further confirmation that a generation of politicians who came to prominence around the former Prime Minister are moving on.

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But his decision to quit will not be welcomed by Gordon Brown this weekend, as he prepares to launch Labour's election campaign, sending out the message that one of his most high-profile MPs is not up for the election.

Speaking in the West Midlands today, Mr Brown will mount what Labour is describing as "Operation Fightback". He will concede he is the underdog but, putting the focus of attention on the Conservatives, will claim that his opponents "haven't moved on".

Mr Purnell resigned as Work and Pensions Secretary last June in protest against Gordon Brown's leadership, arguing that he no longer believed the Prime Minister could beat the Conservatives.

In a statement yesterday, he said he wanted to experience life outside Westminster. "This has been an extremely difficult decision to make. But I have decided that I no longer wish to be an MP," he declared.

"I have spent all my working life in or about Westminster. And while this has been a huge privilege, I've realised I don't want to have spent all my life in frontline politics."

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He was among the Labour MPs most associated with the Blair project. He began his political career in Blair's office, working as the former Prime Minister's main speech writer.

He then became a Downing Street special adviser in 1997. As Work and Pensions Secretary, he proposed a new benefits regime, aimed at getting more people on incapacity benefit back into the job market.

Briefly seen as a potential future leader at the time, Mr Purnell was more commonly viewed as a key player, along with Miliband, in keeping the Blairite wing of the party alive.

Party allies expressed their deep disappointment with the decision yesterday. Glasgow Labour MP Tom Harris said the move was "a blow for those of us who saw him as a future party leader and prime minister".

He added: "I don't really blame him for making the decision which he believes is the right one for him. He's a friend and I wish him well. I would have preferred him to be a member of the Commons after the election and to see him reinstated around the Cabinet table."

Business Secretary Lord Mandelson said: "I'm very sad about James. But I realise that, given his age, he is looking to his future and wondering what he wants to spend the rest of life doing, and has opted for a different life. Politics and parliament will be poorer for that and I regret his decision very much."

Mr Purnell's exit makes the likelihood of a Miliband succession less likely after a possible election defeat. It boosts the chances of other challengers such as schools secretary Ed Balls and deputy leader Harriet Harman, and Ed Miliband, David's brother.

Mr Purnell nearly brought down Mr Brown as leader last year when he quit, but his decision to go failed to trigger other Cabinet resignations. Mr Miliband revealed that he opted not to follow his friend out of the door.

Mr Purnell had long become disillusioned with life as an MP, allies add, saying that the expenses affair – which brought the reputation of MPs to an all-time low – had confirmed his view that a change of career was required.

He is planning to continue a project with think-tank Demos. Friends say he also is planning to re-train as a community organiser while doing some teaching.

One friend said: "Increasingly James has been thinking about the Labour Party and the labour movement and he's moved to a different position where, for instance, he supports a living wage (of 7 an hour], but that was something that when he was a Cabinet minister he said he opposed. He's moved a very long way … but is not leaving politics."

He will also consider doing more teaching shifts at a comprehensive in Kilburn, north-west London, after doing one earlier this month.