David Miliband, who lost a 2010 leadership battle for Britain's opposition Labor Party to younger brother Ed, is quitting politics to head up the International Rescue Committee (IRC).
Miliband, 47, a former British foreign secretary, said he was standing down immediately as a lawmaker to head the New York-based humanitarian organization.
According to the UK Independent, David Miliband was favored to replace Gordon Brown, who resigned in 2010, but lost to his younger brother, who was bolstered by trade unions, at the last minute
Tensions in the family reportedly persist over the contest.
As Labor has gained ground over Prime Minister David Cameron's Conservative party in the polls, David has declined offers by his brother to be more involved in Labor's bid for the 2015 elections.
He returned to the back benches, saying that he wanted to give "Ed the space and at the same time the support he needed to lead the party without distraction."
While saying he would personally miss his brother and that "British politics will be a poorer place without David," Ed Miliband said in a statement:
"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."
David Miliband said it was time for a "new challenge and a new start," and that:
"The IRC represents a unique chance to put my experience into practice on behalf of some of the least fortunate people on Earth."
He also has personal incentive to move stateside: his wife Louise Shackelton, a violinist with the London Symphony Orchestra, is American and they have adopted two children in the United States.
The Guardian speculated that her career may be a factor in Miliband's decision. It also notes that Miliband had a stint at the Massachusetts Institute of Technology.
Meanwhile, Miliband's family history is also being seen as influential in his decision.
His father, Marxist theorist Ralph Miliband, fled from Brussels to Britain as a teenager when the Nazis invaded in 1940. The Milibands' grandmother survived the German occupation in Poland.
The IRC was set up at the suggestion of Albert Einstein in the 1930s for those fleeing the Nazis.
"Starting in September, this job brings together my personal story and political life."
The Independent quoted shadow Energy Minister Caroline Flint as joking:
"Here's to International Rescue - Thunderbirds are go!"