Former UK foreign secretary David Miliband, the elder brother of the opposition Labour Party leader Ed Miliband, has quit British politics, putting an end to speculation about his leadership ambitions. He will become president of a New York-based humanitarian-aid organisation, the International Rescue Committee, ending a 12-year career as a lawmaker. In a letter to his constituents on his website yesterday, Miliband said the move, while tinged with sadness, was a result of his having to "make a choice about how to give full vent to my ideas and ideals". Ed Miliband beat his brother to the Labour leadership in 2010, prompting a rift between the siblings, according to British media reports. Ed twice offered his brother senior roles in his opposition team, both rejected. The two brothers represent different wings of the Labour Party. David, a close ally of former prime minister Tony Blair, favoured a centrist modernising agenda. Ed, by contrast, has closer ties to the party's traditional labour-union supporters, whose backing ensured his election as leader. "After the leadership election, I felt I could be most helpful to the party on the front line," serving voters in his district in South Shields, northeast England, rather than on his brother's parliamentary team, David Miliband wrote on his website yesterday. "I felt this gave Ed the space and at the same time the support he needed to lead the party without distraction," he wrote. "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." With just over two years to go until the next general election, Labour has a consistent 10 percentage-point lead over Prime Minister David Cameron's Conservatives in opinion polls. "We went through a difficult leadership contest but time has helped to heal that," Ed Miliband said. "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." We went through a difficult leadership contest but time has helped to heal that Ed Miliband He said he hoped that "at some point in the future he can once again make a contribution to British public life". Another Blair ally, Peter Mandelson, who serves as a lawmaker in the upper House of Lords, said he would not rule out a comeback to British politics by David Miliband. Mandelson resigned from the cabinet twice before returning as a minister. "I don't think this is the end," Mandelson told the BBC's World Tonight programme. "I think if I can come back, he can."