David Miliband, whose ambition to lead Labour Party in Britain was thwarted by his younger brother, says he is stepping down from frontline politics.
The former foreign minister, one of Labour's best-known figures, says the party needs a fresh start with its new leader, his brother Ed Miliband.
Mr Miliband, 45, says he will stay on as a member of Parliament but will not stand in an internal election to Labour's leadership team or the shadow cabinet.
"This is now Ed's party to lead and he needs to be able to do so as free as possible from distraction," he said in a letter to a Labour official.
"Any new leader needs time and space to set his or her own direction, priorities and policies."
He said that if he had stayed in Labour's leadership team, he feared there would have been "perpetual, distracting and destructive attempts" to find splits between him and his brother.
Ed Miliband won a party leadership vote by 1.3% with the backing of trade unions. The result was announced on Saturday.
He succeeded former prime minister Gordon Brown who quit after a defeat in the May election that ended 13 years of Labour rule.