David Miliband is poised to quit politics for a charity role in New York, the Daily Mirror reports.
The paper says:
The former Foreign Secretary - and brother of Labour leader Ed - intends to make the shock announcement tomorrow morning.
The Mirror understands he intends to step down with immediate effect, triggering a by-election in May
. . . It is believed he will take up a senior role with the charity International Rescue with immediate effect.
The move means an end to Miliband's long political career, which included a stint as foreign secretary under Gordon Brown. It also triggers a by-election in his seat, South Shields, where he has a majority of more than 11,000.
Since coming second to his brother Ed in the Labour leadership contest in 2010, David Miliband has kept a low profile politically, preferring to meet grassroots groups. He made a rare foray into the spotlight with his essay on "Reassurance Labour " in the New Statesman in February 2012, followed by his guest-edit of the magazine in the summer, featuring Hillary Clinton, Kevin Rudd and other international politicians.
Newsnight's Allegra Stratton reports that:
David Mili talked to Ed M for "little while" re leaving, sources say... now Ed sends him on way with "sadness". But no byelex date set.
— Allegra Stratton (@BBCAllegra) March 26, 2013 
Update 27 March 2013 08:10:
LabourList has got the full text of David Miliband's resignation letter  to the South Shields CLP chair. It sheds a little bit more light on his stated motivations for taking the new job at the International Rescue Committee and stepping down from Parliament.
The organisation was founded at the suggestion of Albert Einstein in the 1930s for those fleeing the Nazis, so given my own family history there is an additional personal motivation for me. I feel that in doing this job I will be repaying a personal debt.
He also touches on the 2010 leadership campaign, and his working relationship with Ed since:
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. After the leadership election, I felt I could be most helpful to the party on the front line, in South Shields and around the country, rather than on the front bench in Parliament. I felt this gave Ed the space and at the same time the support he needed to lead the party without distraction. He has done so with real success, leading a united team that has taken the fight to the Tories. I am very pleased and proud that our shared goal of making this a one-term government is achievable.
Ed Miliband has also put out a statement about his brother's departure this morning. He says:
David is taking an important job running the IRC, a global organisation with stature and reach. I am delighted for him that he has been given this opportunity.
Having spoken to him a lot over the past few months, I know how long and hard he thought about this before deciding to take up the offer. I also know how enthusiastic he is about the potential this job provides.
David has made a huge contribution to our country and the Labour Party over two decades. As head of the Downing Street policy unit, as MP for South Shields, as Environment Secretary where he pioneered the Climate Change Act, and as Foreign Secretary where he won respect and admiration around the world.
As for us, we went through a difficult leadership contest but time has helped to heal that. I will miss him. But although he is moving to America, I know he will always be there to offer support and advice when I need it.
British politics will be a poorer place without David. But his huge talents will be serving people around the world. I hope and believe that at some point in the future he can once again make a contribution to British public life.
As ever, that last bit - "at some point in the future" - appears to leave the door open for a return to politics, should he ever wish it.