Former British foreign secretary David Miliband, who lost the 2010 battle for the leadership of the opposition Labour party to his younger brother Ed, announced on Wednesday he is quitting politics.
Miliband is stepping down as a lawmaker to take up the post of president and chief executive of the International Rescue Committee (IRC), a humanitarian organisation based in New York.
The 47-year-old said it was time for a "new challenge and a new start" after the leadership battle with his brother which is reported to still cause tensions in the family.
Ed Miliband said that "British politics will be a poorer place without David", adding that he would personally miss him.
"We went through a difficult leadership contest but time has helped to heal that," the Labour leader said in a statement.
"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."
As one of the leading lights of the Labour party, David Miliband had been seen as an obvious candidate for the top job after 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.
Ed Miliband, whose party is consistently leading Prime Minister David Cameron's Conservatives in the polls, had left the door open for his brother to be involved but David chose to take a back seat.
In a letter on Wednesday to the chairman of the Labour party in South Shields, the northern English seat he has held since 2001, David Miliband praised his brother's leadership.
He explained he had stayed away from frontline politics to give 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," he wrote.
Miliband also described his excitement about his new job at the IRC, which was set up at the suggestion of Albert Einstein in the 1930s for those fleeing the Nazis.
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 survived the German occupation in Poland.
"Starting in September, this job brings together my personal story and political life. It represents a new challenge and a new start," Miliband wrote.