David Miliband has accused Labour of rejecting all the positives of New Labour under his brother Ed's stewardship. Instead of "building on the strengths and remedying the weaknesses" of the Blair years, it "turned the page backwards rather than turning the page forwards".
Speaking on the day that the EU referendum bill easily passed its first hurdle in the Commons, he also expressed his fears for the British if they vote to leave Europe.
Whoever Labour chooses as its next leader, the party should argue that "even the talk of Britain leaving the European Union is dangerous for Britain, and the reality would be disastrous".
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Miliband's comments came in two interviews, one for CNN – his "most withering critique yet" of Labour's failures under Ed, according to The Guardian - and another for The Times, which has questioned more than 40 people for an investigation into why Labour lost the 7 May election.
On Labour's election disaster:
Miliband told CNN's Christiane Amanpour there were "very clear reasons" why Labour lost on 7 May.
Labour under his brother's leadership had not been trusted on the economy. The party must "catch up with the way Britain has changed, the way politics has changed, and the kind of agenda that needs to be set in an age of economic insecurity".
Instead of turning the pages back, Labour needs "to find again that combination of economic dynamism and social justice that defined the success of the Labour party" under Tony Blair.
"It's 50 years since Labour won a majority at a general election without Tony as leader. It's important to have this in mind."
David, who narrowly lost the Labour leadership to Ed in 2010 because of trade union backing for his more left-wing brother, said he had been "very fearful of the consequences" ever since Ed made his pitch to voters.
The result gave him no pleasure – "There is no consolation in any sense of vindication" – but had left him with a sense of "frustration and anger" because of what a Conservative majority government would mean for Britain.
On Britain leaving Europe:
On the "disastrous" reality of Britain leaving Europe, Miliband, who now heads the Manhattan-based International Rescue Committee, said: "Sitting in New York, it's completely evident to me that no American government would ever take seriously a Britain that has withdrawn from the European Union.
"It's almost like Britain would be resigning from the world."
He said Labour would need to be at the forefront of the campaign to remain in the EU, despite not being in government.
Yet only one of the current five hopefuls in the Labour leadership race, Mary Creagh, says she would be prepared to share a pro-EU platform with David Cameron in the referendum campaign.
Creagh and the others – Andy Burnham, Yvette Cooper, Liz Kendall and Jeremy Corbyn – were answering questions yesterday from members of the GMB union at their annual conference in Dublin.
Three of them – Burnham, Cooper and Kendall – have reached the 35 nominations from fellow MPs required to secure a place on the leadership ballot. With nominations closing next Monday, Corbyn currently has 11 backers and Creagh only five.
