Harriet Harman has admitted that the shock exit poll published at 10pm on election day was "a body blow none of us will ever forget".
Labour's interim leader made the remark in a speech on the party's forthcoming leadership contest.
Labour leadership ballot papers will be issued by the Electoral Reform Society so unions cannot try to skew the votes of their members, Harriet Harman is to say.
Candidates will also be "stress tested" at open, televised hustings in areas of the country where the party failed at the election.
Details of the process are due to be set out by acting leader Ms Harman in a speech at Labour's HQ in central London.
Unite's Len McCluskey has said the organisation may cut ties with the party unless its next leader acts as "voice of ordinary people".Read the full story ›
Jon Cruddas MP - who oversaw the writing of the Labour's manifesto - said the party must "articulate a sense of purpose, vision".
"We have to re-establish what is the purpose of the centre-left political party nowadays in terms of its economic strategy, its social strategy, its approach to questions of democracy, power, citizenship.
"The question of political character is now more important than political policies because people don't know who we are. There isn't that basic sentiment in and around Labour so our candidates sometimes find it difficult to articulate a sense of purpose, vision."
Labour leadership candidate Andy Burnham has said Britain must draw up a "credible" package of measures to tackle immigration or face "sleepwalking" out of the European Union.
I want to make the pro European case in this referendum. It is clear to me that British interest is in staying in the European Union.
But I am warning that we will only be able to win that argument if we have a credible package of reforms on immigration.
The public are asking for this. If we don't deliver it, if David Cameron does not deliver it, then we will be sleepwalking to exit from the European Union and I don't want to see that.
Mr Burnham insisted he is the "change candidate" as he made a bid to outflank David Cameron on Europe.
Labour leadership candidate Andy Burnham has said the Labour government was 'not profligate' but did let the deficit get too high.
Speaking on the Andrew Marr show, Mr Burnham said: "The last Labour government was not profligate. The last Labour government was adopting a careful approach, but we did let the deficit get too large."
Labour leadership front-runner Andy Burnham ups pressure on David Cameron over Europe by demanding an in-out referendum is held next year.Read the full story ›
Labour is facing one of the greatest crises in its history, according to the man who oversaw the writing of the party's manifesto.
Jon Cruddas said whoever takes over as leader from Ed Miliband in the wake of a shocking General Election defeat must be prepared for a rethink on what the party is all about.
Speaking on BBC Radio Four, the MP for Dagenham and Rainham said: "Arguably, it's one of the great crises of the Labour Party's history."
I argued that the 2010 defeat was actually the worst defeat in Labour history since 1918, and the defeat of 10 days ago was much worse - so this is profound.
Jim Murphy launched a broadside attack on Unite boss Len McCluskey as he announced he will resign as leader of the Scottish Labour party.
Mr Murphy said that while trade unionists were "a source of enormous strength and moral purpose" to Labour, the party should not be beholden "to one man."
One of the things about stepping down is that you can say things in public that so many people in the Labour party only say in private. So whether it's in Scotland and the contest to come across the UK. We cannot have our leaders selected - or de-selected - by the grudges and grievances of one prominent man.
The leader of the Scottish Labour party doesn't serve at the grace of Len McCluskey. And the next leader of the UK Labour party should not be picked by Len McCluskey.