Former Cabinet minister James Purnell will stand down from Parliament at the general election, he announced today.
The Stalybridge and Hyde MP, who resigned as Work and Pensions Secretary last June in protest against Gordon Brown's leadership, 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," Mr Purnell said in a statement.
"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.
"I'm looking forward to completing my project at Demos. After that my hope is to continue to contribute ideas to public service and to the Labour Party."
Mr Purnell was often tipped as a future leader of the Labour Party before he quit the Cabinet in an abortive bid to bring down the Prime Minister.
He was a researcher for Tony Blair while studying at Oxford, and became a Downing Street special adviser in 1997, before securing the safe seat of Stalybridge and Hyde in 2001. He was returned with a majority of more than 8,000 at the last general election.
Mr Purnell entered the Cabinet as culture secretary in June 2007 at the age of 37, and was promoted to work and pensions secretary the following January.
He became increasing disillusioned with Mr Brown's premiership, and eventually quit as Labour recorded a disastrous performance in local elections.
In his resignation letter, he called on Mr Brown to "stand aside to give our party a fighting chance of winning".
But Mr Purnell failed to persuade other Cabinet "big beasts" to follow his example, and the Prime Minister survived.
Although the MP has always insisted he is committed to the Labour Party, there have been persistent rumours of defection.
His ideas on reforming benefits were seen as closely aligned with the Tories, and he gets on well with shadow skills secretary David Willetts - a driving force behind many of the Opposition's key policies.