Ex-Labour leader Ed Miliband will sit on a panel of party figures to review its general election failure.
Labour Together, which describes itself as a network of activists from all traditions, is setting up a commission to “map out a route back to power”.
It says the panel will view attempts to pin the blame on a single cause, such as Brexit or Jeremy Corbyn’s leadership, as simplistic and wrong.
Members, focus groups in heartlands, and defeated candidates will get a say.
Former shadow education secretary Lucy Powell will spearhead the review, alongside Mr Miliband and Birmingham Ladywood MP Shabana Mahmood.
Ms Powell said the inquiry would take a “real and meaningful look” at why the party had “fallen short” at four consecutive elections.
“We all have to accept that our offers to the country have been insufficient,” she said.
The commission will be made up of voices from “different Labour traditions” and hopes to capitalise on the “millions of conversations” voters had with Labour campaigners, a spokesman said.
Other confirmed commissioners include Jo Platt, who lost her seat in the former stronghold of Leigh, Greater Manchester, Labourlist website editor Sienna Rodgers and James Meadway, former economic adviser to shadow chancellor John McDonnell.
The panel is also expected to recruit a trade union representative and a local organiser.
“Labour Together claims it will seek to rise above the factionalism it says has coloured much of the party’s politics in recent years,” says BBC political correspondent Jessica Parker.
“But – with a potentially bruising leadership battle looming – reaching a consensus on Labour’s past, as well as its future, could prove no easy task.”
Commissioners intend to:
- interview all 59 MPs who lost their seats this month, as well as defeated candidates in target seats
- analyse election data
- ask organisers, councillors and activists what went right and wrong
- survey members
The party last won an election in 2005 under the leadership of former Prime Minister Tony Blair.
Mr Blair said last week that the “takeover of the Labour Party by the far left” had “turned it into a glorified protest movement, with cult trimmings, utterly incapable of being a credible government”.
Mr Corbyn and Mr McDonnell both said they took responsibility for the defeat in the wake of an election that handed the Conservatives an 80-seat majority.
However, they have defended their policies and pointed to Brexit polarising the country.
The defeat prompted a bitter internal row about the party’s Brexit stance.
Ms Rodgers, of Labourlist, said the review would pore over the results in “an even-handed way, which doesn’t start with blaming one faction, or individual”.