Inquiry will look at reasons for party’s poor showing rather than suggesting replacement for Jeremy Corbyn

A review into why Labour lost the general election will not seek to recommend a particular candidate to replace Jeremy Corbyn, the group spearheaded by Ed Miliband said on Monday.

Labour Together, which is running what it calls a cross-factional commission into the party’s worst election showing since 1935, will assess what went wrong in the December poll.

Miliband, the former Labour leader, and Lucy Powell MP, his campaign manager for the 2015 election, are leading the initiative alongside the MP Shabana Mahmood and representatives from unions, local parties and community groups.

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They will be taking soundings from the leftwing campaign group Momentum and the centre-left group Progress.

Although expected leadership contender Lisa Nandy is a member of Labour Together, the review is not designed to back her campaign, Powell told the Guardian.

“We are making sure that those actively seeking leadership themselves or actively helping the leadership candidates are not involved. [No one will be able to] stop the process so that the answer is, for example, ‘Lisa Nandy’.

“We will not be saying the solution lies with ‘x’ ‘or ‘y’ candidate, but what we are looking for is that each of the leadership candidates welcomes our findings and agrees to take them on board.”

The shadow chancellor, John McDonnell, has said he welcomes the review, which is designed to take in the views of Labour members across the political spectrum. Dozens of former MPs, candidates and members of the current shadow cabinet have already contacted Labour Together to give their points of view, according to Powell.

In a tweet, McDonnell wrote: “I welcome this initiative by Ed Miliband.

“Elections are won and lost by a combination of long-term trends and short-term contingencies, but we must also ensure that Labour remains as a party whose fundamental role is to change the world not just react to it.”

Momentum’s former national coordinator Laura Parker is also involved.

There was criticism on social media after the review launched that Miliband is not well placed to guide the party on how to reform for the future considering his defeat to David Cameron in 2015.

Powell responded by saying Miliband retained his Labour seat in the “red wall” as others fell to the Tories; has been campaigning in the traditional heartland of Doncaster regularly for the past four years; and has as much to offer “as the next person”.

“It’s nonsense,” Powell said of the criticism. “We’ve all got something to contribute at different stages. On that logic, that would mean the only people that have got anything to contribute were those who were around Tony Blair in the mid-1990s, which is also a ludicrous suggestion in that we are now living in a different landscape, post-financial crash, post-Brexit and post-expenses, and the electoral challenge is an entirely different one to the 1990s.

“One of the things I reflected on myself in 2015 was that people who run an election that’s not been successful – it can be quite hard to feed in your own personal reflections.”

Nandy, who is expected to announce in the new year that she will run for leader of the Labour party, is touring heartland seats that voted Tory to try to understand how voters lost their traditional links to Labour.

She visited Ashfield, which had been held by Labour’s Gloria de Piero and a Labour safe seat for decades. “I went to Ashfield to listen to lifelong Labour voters who were heartbroken that they couldn’t vote Labour this time,” she said. “They spoke more powerfully than any politician about how unrepresented they have felt for many years. They have to be heard if Labour is to win their votes again.

“Local Labour activists talked about the need for us to become a party that is much more rooted in our communities, and for us to tell a much stronger story about the difference we make at a local level. I share their belief that we need to empower both our activists and the communities we represent if we are going to win back their trust.”

The Labour Together report will be released in the second half of February, with the leadership election expected to take place in March.