Ed Miliband should not bank on a coalition deal if he fails to win an overall majority in 2015, his brother David Miliband has warned.
The former foreign secretary, who is about to move to New York to begin his new job as head of the International Rescue organisation, said he expected either Labour or the Conservatives would win the next election outright - and the "danger" for his brother is it could be David Cameron who does so.
Figures within the Labour party have spent recent months improving backroom relations with the Liberal Democrats, potentially paving the way for a Lib-Lab deal after the next election.
"I think the conventional assumption we're bound to get a coalition is wrong," David Miliband told BBC1's The Andrew Marr Show.
""I think in the end the British people will take a view. That is a great prize for Labour. The danger is it could be a great prize for the Tories too."
A poll out today by ComRes for the Sunday Mirror put Labour on 36%, up one point, and the Tories trailing by eight points, but up two on last month's numbers, on 28%. Ukip are down one point to 18% and the Lib Dems drop two points to 18%.
"There's a bit too much mathematics going on in the way people are looking at the polls," Miliband added.
"I don't think anyone on either side should be banking on the fact it's bound to be a coalition, I don't see it that way."
David Miliband's departure for the US comes after a week which saw Ed Miliband attempt a major confrontation with trade union leaders, potentially breaking the unions' often decisive influence on the Labour party.
"I've had a sense there was an inevitability about reform," he said, as he voiced support for his brother's moves.
"Around the world, the old structure of political parties are dying. They have to renew themselves by opening themselves up.
"The bolder they are, the more open they are, the more successful they're going to be. And they've got to look like the rest of the country. They're going to have to be thoroughgoing."
He refused to rule out a return to frontline politics in Britain and suggested he could never properly move on from his 2010 Labour leadership election defeat to his younger sibling.
"These things - you can never erase them from memory or history, it's not right to pretend that," Miliband said.
"But me and Ed are brothers for life - that's something you value for life whatever the nature of the circumstances.
"The important this is we've got to never lead our lives by looking in the rear-view mirror.
"There are the [Andy] Murrays of this world and there are the [Novak] Djokovics who come second, and you've got to be gracious when you don't win."
Djokovic is the Serbian tennis player ranked number one in the world by the Association of Tennis Professionals. Although he lost to Murray in last weekend's dramatic Wimbledon final, overall Djokovic leads Murray 11-7 in their head-to-head clashes.