DAVID Miliband has insisted it was his fear the family "soap opera" would take over the real substance of Labour politics that ultimately led him to decide to leave Westminster for a high-profile charity job.

The decision by the former Foreign Secretary to quit as an MP to become chief executive of the International Rescue Committee in New York shocked many colleagues.

David Blunkett, a former Cabinet minister, said he was deeply saddened but others suggested it was inevitable.

Yet New Labour grandees held out the prospect that Mr Miliband might one day return to frontline British politics. Lord Mandelson, the twice-resigned former Business Secretary, said: "I wouldn't say goodbye to David Miliband forever in British politics."

Tony Blair, the ex-PM for whom Mr Miliband worked as head of the No 10 policy unit, said: "I hope and believe this is time out, not time over."

Mr Miliband admitted to wrestling with "this very, very hard" decision for some time but insisted he did not want the choice for voters to be distracted by the "minutiae of the relationship between two brothers who fought a leadership election campaign".

He added: "I didn't want to become a distraction from that central task."

The human drama that was the battle of brother against brother had echoes of the Blair-Brown relationship, where the older political brother had expected confidently to get the party inheritance but had to watch bitterly as it went instead to his younger sibling.

Ed Miliband said: "British politics will be a poorer place without David but his huge talents will be serving people around the world. I hope and believe that at some point in the future he can once again make a contribution to British public life."

Mr Miliband's departure now leaves the prospect ahead of a by-election in South Shields, but the seat has been Labour's for almost 100 years.

In his resignation letter, Mr Miliband said: "Has it been hard for me to accept that I can best help the Labour Party by not just giving the space between the front bench and the back bench to Ed but the space between the front bench and 3000 miles away? Yes."

Some Blairites might believe David could become the "king over the water" but the precedent is not a happy one.