Former British foreign secretary David Miliband said on Wednesday he was quitting politics, bringing an end to a soap opera of sibling rivalry that has dogged the opposition Labour party.

The 47-year-old is stepping down as a lawmaker to become head of the International Rescue Committee (IRC), a humanitarian organisation based in New York, in September.

The decision comes two and a half years after Miliband lost out to his younger brother Ed in a battle for the Labour leadership, in a shock defeat which remains a source of tensions between the pair.

Miliband said the job represented a "new challenge and a new start" in the United States, birthplace of his wife, violinist Louise Shackelton, and their two adopted children.

Ed Miliband, 43, said that "British politics will be a poorer place without David", adding that he would miss him.

"We went through a difficult leadership contest but time has helped to heal that," the Labour leader said in a statement.

As one of the leading lights of the Labour party, David Miliband had been seen as an obvious candidate for the top job after Labour prime minister Gordon Brown lost the general election in 2010.

But his failure to seize repeated chances at the leadership before that had lost him support, and his brother pipped him to the post with the support of the trade unions.

In a letter on Wednesday to the chairman of the Labour party in South Shields, the seat in northern England he has held since 2001, Miliband praised his brother's leadership of the party.

He explained he had stayed away from frontline politics to give Ed "space" and said he had led the party "with real success, leading a united team that has taken the fight to the Tories" of Prime Minister David Cameron.

Tributes and messages of regret at Miliband's departure poured in from former Labour colleagues, while former US president Bill Clinton offered his congratulations on the new job.

"I have known David almost 20 years. He is one of the ablest, most creative public servants of our time," Clinton said in a statement on the IRC website.

Miliband struck up a close relationship with Clinton's wife Hillary while she was US secretary of state and he was foreign secretary between 2007 and 2010.

Former Labour prime minister Tony Blair, who hired David Miliband as his head of policy while in opposition and gave him his first ministerial job, said he hoped his political career was not over.

Miliband "remains one of the most capable progressive thinkers and leaders globally. I hope and believe this is time out not time over," Blair said in a statement.

Since leaving frontline British politics Miliband has earned about £1 million (1.2 million euros, $1.5 million) with jobs in academia, business and public speaking, The Times reported.

Miliband said his new job "brings together my personal story and political life", noting the IRC's roots as an organisation for those fleeing the Nazis in the 1930s. Today it responds to humanitarian crises across the world.

His father, Marxist theorist Ralph Miliband, was born in Brussels and fled to Britain as a teenager when the Nazis invaded in 1940, while his mother Marion survived the German occupation in Poland.