Labour must look "deep in our souls" to understand why it lost the election, acting leader Harriet Harman has said - while warning the party not to tear itself apart arguing over it.
She told MPs gathered at Westminster for the first time since the election that she had commissioned a "forensic analysis" of what went wrong and warned there was "frustration" at the amount of "commentating" that had been going on within the party.
She spoke as ex-leadership candidate David Miliband attacked the "backwards" campaign led by his younger brother - and many party grandees lined up to air their thoughts on why the party had lost.
Harman told the MPs: "We have got to look deep in our souls, but we shouldn't open our veins."
The older Miliband has again failed to rule himself out entirely as a future Labour leader in his first interview since the party's crushing defeat and his brother's resignation.
David Miliband, who was predictably accused of oh-so-cryptically suggesting a return to British politics in a series of tweets on Friday, was highly critical of his younger brother, accusing Ed Miliband of allowing himself to be portrayed as "moving backwards from the principles of aspiration and inclusion".
When asked whether he would seek to return to lead his old party, David Miliband again phrased his answer in a way that did not rule out a future bid, a habit that has marked his time in New York and infuriated many former colleagues.
Miliband on the BBC
Miliband, 49, who resigned as an MP in 2013 to take over as CEO of the charity International Rescue Committee, said: "I'm clearly not a candidate in this leadership election, the commitment I have to the job I've got doesn't change."
He added: "The passion I've got for Britain and for Labour is undimmed. It's as strong as ever. But it's clear that I'm not a member of Parliament. I took that decision not to be part of a soap opera over the last five years.
"I wanted Labour to be able to have that debate in a clear way, make its case in a clear way without any sense of distraction from me. And I hope now people can listen to what I say as someone who's trying to contribute but obviously isn't party to the race that's going on."
Keen observers of Westminster noted his obvious failure to rule out a return to Westminster.
David Miliband criticised his brother's campaign, in terms that he had "waited a while to get off his chest", according to Ed Miliband biographer Mehdi Hasan.
David Miliband echoed the Blairite message other party grandees have been pushing - that the party has to move to the right and appeal to a broader range of aspirational voters.
The older Miliband said: "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."
He added: "There's absolutely no point in blaming the electorate. Any suggestion that they didn't 'get it' is wrong. They didn't want what was being offered."
He added his party needed to talk about "how you have a dynamic economy, how you achieve social reform, how you modernise policy, how Britain needs to play a part in the international system".
He added: "That needs to be done in a way that embraces people rather than divides them. The lesson of the election, the lesson of the voters is that we failed to do that in 2010 and in 2015."
When asked about his 42-year-old brother, Miliband said he was "very happy to say we remain in touch".
He said: "I think that many of the attacks on him were unpleasant and unfair and I think he dealt with them with enormous dignity and courage.
"I've always said that I'll keep our private conversations private, but I've also always said that we remain brothers for life and that's something that has to be kept."
At tonight's meeting in Westminster, Harman told Labour MPs that the party was considering three approaches for staging the contest to succeed Ed Miliband - with a final decision to be taken by the ruling national executive on Wednesday.
The options are short campaign with the result decided on July 31, a longer campaign with the new leader chosen one or two weeks before the party conference in September, or using conference as a final hustings with a ballot after that.
Potential leadership candidate and Shadow Education Secretary Tristram Hunt told ITV's The Agenda that criticism from all corners was needed because the party was "in a real hole".
He added: "Everyone who loves the Labour party and wants it to return to government should get involved - that's David Miliband in New York, that's Peter Mandelson, that's Tony Blair, that's John Prescott. Everyone. Because we are in a real hole."
Asked whether he would run to be leader, he said: "I haven't decided. I am talking to colleagues about it.
"The issue is about the ideas and the lessons the philosophy and the politics. Leadership is part of that but it's having the principles behind that.
"Having someone to make those arguments and the nature of those arguments is incredibly important. If it's me, fine, if it is others fine. It has to be made."