Senior Labour figures will launch a post-election autopsy into how the party lost 59 seats and handed Boris Johnson's Conservatives a large majority.
Ed Miliband, leader from 2010-15, is one of those setting up a commission to "learn the lessons" of this month's snap poll by "rising above the factional infighting".
He will be joined by Lucy Powell, a Labour MP who ran the party’s unsuccessful 2015 election campaign, backbencher Shabana Mahmood, a trade union representative, and an MP who lost her seat - Jo Platt.
Shadow chancellor John McDonnell's economic adviser James Meadway and editor of the website Labourlist Sienna Rodgers are also on board.
The commission is promising to take a "balanced view of what happened in the campaign" by taking evidence from defeated Labour candidates, holding focus groups in heartland seats the party lost and doing "deep, objective analysis" of election data.
It wants to map out a route back to power for Labour by working with different factions, including Momentum, Labour First, Open Labour, Progress.
Labour so far has been in opposition for nine years and suffered its worst result since 1935 just 11 days ago.
Commissioner Ms Powell called it "deeply disappointing".
"We have lost the last four elections and we all have to accept that our offers to the country have been insufficient," she said.
"We should have taken the time to understand our losses previously.
"It's now profoundly important for the future of our party and country that we take a real and meaningful look at why we have fallen short.
"This inquiry gives us the opportunity to listen to members, candidates and the public and I hope our whole movement takes it in the spirit it is offered and takes part."
It comes as an anonymous survey for candidates, activists and others involved in the campaign to share their concerns has been posted online.
One Labour MP told Sky News: "This commission of people who lost the 2015 campaign, which has appointed itself, is unlikely to deliver the brutal hard truths the party needs to hear.
"We need an independent firm to do a proper job, including hearing from people who have been involved in winning and losing over the years.
"From what I've seen on this survey there aren't specific, detailed questions about voters' views on the leader; or the organisational failures in the campaign."
It is understood that the Labour Party will launch its own rival inquiry into the 2019 campaign.
But efforts to unify Labour still seem some way off.
MP Siobhain McDonagh confronted new colleague Sam Tarry on Twitter for claiming "the Tories are clearly scared" of peoples' right to strike and vote.
"I am sure the Tories are terrified," she wrote.
"They won the Brexit Bill vote by 124 votes. They have a Commons majority of 80. While Jeremy Corbyn remains in place until March & one of the leadership contenders asks a Stalinist to run her campaign! Yep they're terrified!"
Jeremy Corbyn has pledged to remain leader until his successor is elected, with a timetable expected to be published in the new year.