David Miliband is planning to leave Parliament to move to the US to work for a charity, a close friend has confirmed to the BBC.
He is going to become head of the International Rescue Committee in New York, the BBC understands.
Mr Miliband, 47, a former foreign secretary, was beaten to the Labour leadership in 2010 by his brother Ed.
The South Shields MP has taken a back seat since, opting not to join his brother’s shadow cabinet.
There had been widespread rumours that he was set for a return to the opposition front bench, with Ed Miliband insisting recently that “the door is open”.
However, the Daily Mirror reported earlier that Mr Miliband was planning to announce his decision on Wednesday morning.
The BBC understands Mr Miliband, nicknamed “Brains” by former Number 10 spin chief Alastair Campbell, will take up the job of CEO of the International Rescue Committee.