If what could be either a cruel blow or a huge boost to his brother, David Miliband will be stepping down as an MP on Wednesday, to take a job at a top charity in New York.
The role, with the charity International Rescue, is a "dream role" for the former Foreign Secretary, who will move his young family to his wife Louise Shackleton's native US.
In his resignation letter, Miliband wrote: "In every job I have done, in and out of Parliament, I have sought to make a difference to the disadvantaged and vulnerable.
"The IRC does this on a daily basis and a large scale for some of the most desperate people in the world.
"Of course it is very difficult for me to leave Parliament and politics, friends and colleagues. As you know, I see every day the damage this shocking government is doing to our country, and passionately want to see Labour back in power.
"I will forever be Labour. But after writing two election manifestoes in 1997 and 2001, and serving as a Minister for eight years, I now have to make a choice about how to give full vent to my ideas and ideals.
"I hope you will understand that the opportunity to lead the IRC represents a unique chance to put my experience into practice on behalf of some of the least fortunate people on Earth."
A source close to Miliband, who has been MP for South Shields since June 2001, told The Mirror he will step down on Wednesday. “David has landed a dream job in charge of a charity in New York and wants to take it. But it means leaving politics.”
A senior Labour source told The Huffington Post UK they were "genuinely shocked" by the news, but added: "It's a major loss for the party, but I don't think it'll affect Labour's electoral chances. It may be that he just wanted to make a clean break before 2015."
Since losing the leadership election to his brother by a margin as thin as a cigarette paper, Miliband has repeatedly turned down offers of jobs in the Shadow Cabinet, including the job as Shadow Chancellor.
His party's MPs were quick to pay tribute to the man a majority had hoped would be Labour leader.
Labour MP Stella Creasy said it had been "an honour" to work with the former Foreign Secretary, and fellow Labour MPs Jamie Reed and Toby Perkins also expressed gratitude to Miliband for his work in politics.
David Lammy, the Labour MP for Tottenham, confirmed his close friend was indeed departing the Commons.
Labour MP Yasmin Qureshi, who backed the elder Milband for Labour leader, told HuffPost UK: "It's a really sad loss for British politics, the loss of an incredibly intelligent, principled and honest man. We need more people like him. I do wish him every success."
Miliband's resignation will trigger a by-election in his South Shields constituency. The MP, who is vice-chairman and non-executive director of Sunderland Football Club, secured a majority of 11,109 at the 2010 general election with 52% of the vote.
Earlier this month, in an interview with the Times, Miliband hinted he was looking for fulfilment beyond the front benches.
“I still think you have got to do both,” he told the paper. “I can speak about what I think about the welfare Bill in Parliament, which I did, but I can also try to make a difference on the ground. So much of politics is people saying yada yada, but what’s it actually going to mean?
“It sounds a bit like a soundbite but ‘front line not front bench’ does sum it up.”Suggest a correction