David Miliband has admitted he is resigning as a Labour MP to end the "soap opera" surrounding his relationship with his brother Ed.
The former foreign secretary is quitting his South Shields seat and moving to New York to take up the position of president and chief executive of the International Rescue Committee.
Mr Miliband said he wanted to end any fascination over the relationship with his brother, but resisted ruling out a return to Westminster politics.
Labour leader Ed said British politics will be "a poorer place" without David, and admitted the bruising leadership battle which pitted the two brothers against each other had been "difficult".
"Time has helped to heal that," he added. "I will miss him."
In a letter to his constituency party chairman in South Shields, 47-year-old David - who was long regarded as the brightest Labour star of his generation and the party's most likely future prime minister - said it was "very difficult" for him to be leaving politics in the UK.
He said he still "passionately" wanted to see Labour returned to power, and praised Ed's leadership for making it "achievable".
"I've wrestled with this very, very hard and I have tried to make a decision... that is right not just for me but is right for the Labour Party as well," he told the BBC.
"I want it to be the vision Ed has versus David Cameron's vision. I didn't want to become a distraction from that central task, I didn't want the soap opera to take over the real substance of what needs to be done."
The announcement prompted praise for David's political career from across the parties as well as former colleagues, with his former boss in government Tony Blair saying: "He is obviously a massive loss to UK politics. He was the head of my policy unit and then a truly distinguished minister in the Government and remains one of the most capable progressive thinkers and leaders globally. I hope and believe this is time out not time over."