Miliband quits (no, not that one!): 'British politics will be a poorer place now David is stepping down', says brother Ed

Ed Miliband said today that British politics will be a “poorer place” after his brother David confirmed that he is standing down immediately as an MP to head a New York-based charity.

David, the former Foreign Secretary, is to move to the United States to become chief executive of the International Rescue Committee (IRC), which provides humanitarian relief in more than 40 countries. He will quit as MP for South Shields, causing a by-election in the safe Labour seat he has represented since 2001.

David was favourite to win the Labour leadership in 2010 when Gordon Brown resigned but was pipped at the post by his younger brother Ed.

In a letter to his constituency Labour party, David said it would be "very difficult" for him to be leaving UK politics. He "passionately" wanted to see Labour returned to power and praised his brother’s leadership for making victory at the 2015 election "achievable".

David said he had returned to the back benches in 2010 to give “Ed the space and at the same time the support he needed to lead the party without distraction.” He went on: "I will forever be Labour. But after writing two election manifestos in 1997 and 2001, and serving as a minister for eight years, I now have to make a choice about how to give full vent to my ideas and ideals.”

He told his local party: "I hope you will understand that the opportunity to lead the IRC represents a unique chance to put my experience into practice on behalf of some of the least fortunate people on Earth."

David’s wife, the violinist Louise Shackelton, is American and they have adopted two children in the United States.

Ed Miliband admitted the brothers’ relationship had been strained by the 2010 Labour election. "We went through a difficult leadership contest but time has helped to heal that. 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."

Blairites insisted that David’s departure at the age of 47 did not necessarily spell the end of his political career. Tony Blair, said: “I hope and believe this is time out not time over." Mr Blair added: “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.”

Lord Mandelson, the former Cabinet minister who made a surprise return to serve under Mr Brown, said: “Never say never. I wouldn't say goodbye to David Miliband forever in British politics.” He added: “ If I can come back, he can.”

Recalling the nickname "Brains" given to David Miliband by the former Downing Street spin doctor Alastair Campbell, the shadow Energy Secretary Caroline Flint joked: "Here's to International Rescue - Thunderbirds are go!"