David Miliband has announced he will not seek election to the shadow cabinet, saying Labour is now his brother's to lead.
Speaking to the BBC the shadow foreign secretary said he thought he could best support Labour's new leader from the backbenches.
"I think I can give him the freedom and the space to drive the party forward in the way that he sees fit without any of the distractions that can come with having your older brother sitting next to you," he said.
"Ed needs a clean field to make sure that he's able to lead the party forward as he sees fit without anyone saying that there are distractions because I'm there.
But he refused to rule out a return to the front bench at some point in the future, telling the BBC's political editor Nick Robinson: "I'll always make the decision I think is right for the party and the country".
"The great thing about elections is in the end someone wins and someone loses," he added. "Magnanimity in defeat is important".
"The right thing for me now and for the foreseeable future is to support Ed from the back benches"
Speaking outside the Midland Hotel at the Labour Party conference in Manchester, Ed Miliband said his brother had made a "thoughtful and gracious decision".
Labour's new leader said the party would have had a "stronger team" if the shadow foreign secretary had chosen to stay on the front bench, but he said it was the "right decision for him".
And he said the "door is always open" for David to return to the frontline.
"He's a massive talent for our party and indeed for our country," he said.
Ed Miliband said Labour would have had a "stronger team" if the shadow foreign secretary had chosen to stay on the front bench, but he said it was the "right decision for him".
"I'm delighted to be leader of this party, but I'm disappointed for him, that is the paradox," he said.
He said that he belived his brother had a "big future" ahead of him and looked forward the possibility of him returning to government one day.
In a letter to his constituency chairman Alan Donnelly, David Miliband said Labour needed a "fresh start".
"On the day that nominations closed for the Shadow Cabinet, I think it right to explain to you and party members why I think I can best support him (Ed) from the back benches,"
"The party needs a fresh start from its new leader, and I think that is more likely to be achieved if I make a fresh start. This has not been an easy decision, but having thought it through, and discussed it with family and friends I am absolutely confident it is the right decision for Ed, for the party, and for me and the family."
He continues: "This is now Ed's Party to lead and he needs to be able to do so as free as possible from distraction.
"Any new leader needs time and space to set his or her own direction, priorities and policies. I believe this will be harder if there is constant comparison with my comments and position as a member of the Shadow Cabinet.
"This is because of the simple fact that Ed is my brother, who has just defeated me for the Leadership. I genuinely fear perpetual, distracting and destructive attempts to find division where there is none, and splits where they don't exist, all to the detriment of the Party's cause."
The Labour Party conference has been dominated by speculation about whether David Miliband would serve under his brother following his defeat in the leadership election.
The shadow foreign secretary left the Labour Party conference in Manchester yesterday afternoon having sat stony faced through sections of his brother's leadership speech.
And Ed Miliband's keynote address was somewhat overshadowed after film emerged of David rebuking Harriet Harman for applauding the new Labour leader's denunciation of the Iraq War.
"You voted for it, why are you clapping?" David Miliband asked Labour's deputy leader.
Harman responded: "I'm clapping because he is the leader. I'm supporting him."
David Miliband's rise from Downing Street backroom boy to Foreign Secretary in six years had been dramatic. Though seen as the young heir to Blairism by some sections of the Party, he refused to stand against Brown after Blair’s departure in 2007; and was rewarded with his appointment as the youngest Foreign Secretary since David Owen in 1977.
He parachuted straight from the prime minister's Policy Unit (where he was known as “Brains”) into South Shields at the last minute in 2001.
Within four years Miliband was in the Cabinet, where he spent much of his time denying that he had any intention of challenging for the leadership, despite constant wishful talk in the media and among Brown’s opponents.
Nobody believed the powerful New Labour thinker would remain a humble Tyneside backbencher for long. Within a year, at the age of thirty-six, he was in the government, promoted straight to the middle-ranking position of schools minister.
In 2004 he moved to the Cabinet Office to be Alan Milburn's number two, working on policy for the general election. After the election he added some intellectual stiffening to the Office of the Deputy Prime Minister, as minister for communities and local government.
He now had a seat at the Cabinet table, and before long had a Department of his own to go with it. In 2006 he succeeded Margaret Beckett as Secretary of State for Environment, Food and Rural Affairs. A year later he succeeded her again, to become Foreign Secretary at the age of forty-one.
Labour will announce the results of its shadow cabinet elections on October 7.


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