David Miliband walks away from politics – for now

David Miliband is stepping down as an MP with immediate effect so he can take a high profile charity job in New York.

The man widely expected to succeed Gordon Brown as leader of the Labour party will instead become chief executive of the International Rescue Committee (IRC), a refugee charity with 12,000 staff employed in 40 countries.

The former foreign secretary has been the subject of media speculation since he narrowly lost the Labour leadership to his brother.

The Blairite minister lost the leadership race to his brother by the narrowest of margins, coming in at 49% to Ed Milband's 51%.

He won the backing of a majority of party members and MPs, but lost among trade unions. Since then, he has made just under £1 million on top of his MP's salaries.

He was always admired in American political circles, with Barack Obama's first secretary of state Hillary Clinton famously admitting to a crush on him when they worked together.

The move comes just as relations with his brother were thought to be improving, with some commentators suggesting he could be about to make a move back into a frontline position.

His decision to quit as an MP suggests he is confident his brother will win the general election. A loss in the 2015 poll would have offered him an opportunity to try again for Labour leader.

In a letter to his local party chair, Alan Donnelly, Miliband praised his brother Ed for having led "a united team that has taken the fight to the Tories".

He added: "I am very pleased and proud that our shared goal of making this a one-term government is achievable."

Westminster journalists were surprised by the decision, which appeared to be at odds with his active work on the backbenches so far this year.

Ed Miliband revealed this morning his brother had been considering walking away from British politics altogether for some months, however.

"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," he said.

"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."

David Miliband becomes the second MP to quit and move to New York this parliament, after former Corby MP Louise Mensch resettled across the Atlantic to spend more time with her family.

His eparture will spark an immediate by-election in South Shields, but the solidly Labour constituency is unlikely to see an electoral upset.

It is the only constituency never to have had a Tory MP since the Great Reform Act of 1832.

"We will all miss him. We will all miss his energy and contribution. I hope at some point in the future David will once again make a contribution to British political life," former Cabinet colleague Tessa Jowell told the Today programme.

"This is a dream job for him. The International Rescue Committee is a humanitarian organisation that works all around the world. It was founded to support refugees escaping the Nazis, and for David there's a very important connection because both his parents were refugees."

Miliband wrote: "I feel that in doing this job I will be repaying a personal debt."