DAVID Miliband has waded into Labour's election post-mortem process, claiming the party lost because his brother Ed retreated from the principles of aspiration and inclusion that had helped New Labour win three successive victories.

The former Foreign Secretary, who runs a global charity in New York after quitting British politics when his younger brother beat him to the Labour leadership in 2010, said the voters "didn't want what was being offered" by Labour.

His criticisms echoed those of Tony Blair and Lord Mandelson.

Mr Miliband ruled himself out of the party's leadership contest although it seemed clear he might one day return to frontline British politics.

"What is vital is there is absolutely no delusion about what happened, why it happened, and the scale of the challenge that exists not just in England and Wales but in Scotland as well," said the ex-Cabinet Minister.

He went on: "Both in 2010 and 2015, Gordon(Brown) and then Ed allowed themselves to be portrayed as moving backwards from the principles of aspiration and inclusion that are at the absolute heart of any successful progressive political project.

"The answer is not to go back to 1997, it's to build on the achievements and remedy the weaknesses but never to end up in a position where the electorate think you are going backwards rather than addressing the issues of the future."

In another stinging criticism, Alistair Darling, the former Labour Chancellor, insisted Labour was simply not convincing in the policies it put forward.

"We did not have an economic policy," declared Mr Darling, who stood down from his Edinburgh South West seat at the election.

The former leader of the No campaign admitted during the campaign people avoided making eye contact with him on the doorstep. "When that happens you are done for," he said.

"The last time that happened was in 1992. I'm afraid we are back there in political terms; only worse because the future of our country, of Scotland in my case, the United Kingdom as well, is at stake."

Tomorrow, Labour's ruling NEC meets to discuss the election defeat.

Harriet Harman, the acting leader, argued there was no snap answer to Labour's most damaging election defeat in more than 20 years and revealed an analysis had been commissioned about why the party's pre-election soundings were so wrong.

"The idea that a few days after...we all suddenly know the answer," she declared.

Ms Harman added: "We've got to have a process of deliberation, we've got to look at the facts because what happened in Scotland, for example, is probably rather different - we lost dozens of seats there - than what happened in different parts of England."

The acting leader announced the line-up of her new Shadow Cabinet, filling gaps left by the party's disastrous showing in the election.

Key promotions go to Chris Leslie, who becomes Shadow Chancellor in the wake of Ed Balls's surprise defeat in Morley, and Hilary Benn, the former Shadow Communities Secretary, who becomes Shadow Foreign Secretary, taking up the role previously held by Douglas Alexander, who lost his Paisley seat.

The only surviving Scottish Labour MP, Ian Murray, becomes Shadow Scottish Secretary.

Other senior roles are unchanged from Mr Miliband's team with Yvette Cooper keeping the home affairs brief, Andy Burnham health, Chuka Umunna business, and Tristram Hunt education.

All four are considered likely contenders in the forthcoming leadership battle. Whenever the new leader is elected, there will be a new Shadow Cabinet team.