A group of MPs and other Labour members intended to represent opinions from across all wings of the party have launched a formal inquiry into its 2019 general election defeat and the lessons that can be learned.

The panel, including MPs Lucy Powell, Ed Miliband and Shabana Mahmood, as well as a former adviser to the shadow chancellor, John McDonnell, and representatives from unions, local parties and community groups, hopes to report by the end of February, before a new leader is chosen.

Organised by Labour Together, a network launched in 2015 to gather ideas from across the party, the inquiry aims to get beyond “simplistic takes” so far offered, for example blaming the loss entirely on Jeremy Corbyn or on Brexit.

The news comes amid an apparent Christmas hiatus in leadership manoeuvrings, with only two candidates so far having formally declared their intention to stand, but up to half a dozen more mooting possible bids.

Last week, Emily Thornberry, the shadow foreign secretary, set out her pitch for the leadership, saying she had warned that backing an election in which Brexit was a key issue would be “act of catastrophic political folly”.

The next day, Clive Lewis, a shadow Treasury minister, declared his candidacy with a pitch towards the left of the party, promising also to increase members’ role in policy-making.

Widely expected to stand but not yet formally declared are the two favourites: Keir Starmer, the shadow Brexit secretary, and Rebecca Long-Bailey, the shadow business secretary and the apparent official choice of the Corbyn/McDonnell wing of the party. She has so far remained silent since the election.

Others who could stand include the Wigan MP Lisa Nandy, Tottenham MP David Lammy, and home affairs committee chair Yvette Cooper. Angela Rayner, the shadow education secretary, had been tipped as a candidate but will reportedly stand instead for deputy leader, on an unofficial joint ticket with Long-Bailey.

The inquiry, styled as “an independent commission to learn the lessons of the 2019 Labour general election campaign”, is separate from any of the leadership campaigns, and hopes to involve groups within Labour such as the Corbyn-backing Momentum and the centre-right Progress faction.

Panel members will take evidence from defeated candidates in seats lost by the party, and from those in target constituencies. The inquiry will also hold focus groups and “listening events” with the public in seats that were lost; speak to organisers, councillors and activists; carry out a survey of party members, and analyse election data.

Organised jointly by LabourList, the independent news website focused on party issues, other panel members include LabourList editor Sienna Rodgers, James Meadway, a former adviser to McDonnell, Marcus Roberts from the polling organisation YouGov, representatives from Common Knowledge, a workers’ co-operative, and others yet to be announced.

The aim is to “take a balanced view of what happened in the campaign, aiming to bring the Labour movement together so that it can come to a collective view of what needs to change to make the party electable again”, according to a statement announcing the plan.

Powell, the Manchester Central MP, said: “We have lost the last four elections and we all have to accept that our offers to the country have been insufficient.

“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.”