The starting gun has been fired on the race to succeed Jeremy Corbyn as Labour leader, as the party became engulfed in a bitter row about whether its Brexit position or leadership were mainly to blame for last week’s election disaster.

Labour confirmed on Sunday night that Corbyn had asked for a leadership process to elect his successor by the end of March, meaning he will carry on in post and opposing Boris Johnson across the dispatch box for more than three months.

The party’s general secretary, Jennie Formby, wrote to the national executive committee to say a full timetable for the leadership contest would be agreed, with a recommended start date of 7 January.

John McDonnell, the shadow chancellor, and Richard Burgon, the shadow justice secretary, threw their weight behind their long-term ally Rebecca Long-Bailey for the top job while blaming Brexit for the party losing support across the north and Midlands.

Labour leadership contest: who are the runners and riders? Read more

With endorsement from the Corbynite wing of the party, Long-Bailey is now the favourite for the job even though she has not formally declared her candidacy.

But a large field of mostly female contenders – including Lisa Nandy, Jess Phillips, Angela Rayner, Yvette Cooper and Emily Thornberry – are also mulling whether to throw their hats into the ring, with wildly different assessments of what went wrong for the party.

Diane Abbott, another leading Labour woman, ruled herself out of the race, telling the Guardian it was important that whoever was the new leader was able to stand up to “all the attacks that will come from Johnson and Donald Trump”.

Keir Starmer, the shadow Brexit secretary, has long been considered another favourite but many MPs are keen to see a woman lead the party for the first time in its history. Both Starmer and Thornberry played a key role in pushing Labour towards a second referendum position on Brexit and both represent north London seats, and some MPs are already arguing against either getting the job for those reasons.

Caroline Flint, a former minister who lost her Don Valley seat, said Long-Bailey and Nandy were the only candidates worth considering as they had not been complicit in pushing the party towards a more remain position. Speaking on Sky News, she then launched an attack on Thornberry in particular, claiming the shadow foreign secretary had told a colleague: “I’m glad my constituents aren’t as stupid as yours.”

Thornberry, who has not yet confirmed whether she is running, hit back, saying: “This is a total and utter lie. I have never said this to anyone, nor anything like it, and I hope, needless to say, it is not something I would ever think.” She is understood to be consulting lawyers about the allegation.

McDonnell said on Sunday that he wanted to see a woman lead the party, and preferably someone from outside London. He said Long-Bailey “could be a brilliant leader” and highlighted others including Dawn Butler, the shadow equalities minister, and Rayner, who could be part of a “terrific team”.

“You know my view, I think Becky Long-Bailey’s done a great job. But you’ve got a team there. You’ve got a real team,” he said.

McDonnell, who ran Labour’s election campaign, issued the first big apology from a senior party figure for its performance.

He told the BBC’s Andrew Marr Show: “It’s on me. Let’s take it on the chin. I own this disaster, so I apologise. I apologise to all those wonderful Labour MPs that lost their seats, who worked so hard. I apologise to all our campaigners. But most of all, I apologise to those people who desperately need a Labour government. And, yes, you know, if anyone’s to blame it is me, full stop.”

Play Video 0:34 John McDonnell: 'If anyone’s to blame it’s me, full stop' – video

Corbyn has not apologised for the election defeat but he has said he is “very sad” about the result. He released a video on Sunday in an apparent attempt to defend his policies and cement his legacy.

“Make no mistake, Labour is the greatest force for progressive change this country has ever known. And though this wasn’t our moment, our time will come,” he said. “Over the last four years we have built a new movement which isn’t going away. For those that feel disheartened and feel like giving up, I say stay and fight for a better society.”

Corbyn made no reference to his intention to step down but McDonnell confirmed they would both leave their posts within the next eight to 10 weeks.

There is pressure from some Labour figures for Corbyn to leave sooner. Harriet Harman, the former deputy leader who stood in when Ed Miliband lost in 2015, said Corbyn should go and had shown “no willingness to understand why ⁦[Labour] suffered this catastrophic defeat”.

It is understood some potential leadership candidates want a timetable that gives them longer to put their cases to the public, but there are also concerns on the non-Corbynite wing that the central party could design the contest to help favoured candidates.

The current rulebook suggests people will be able to sign up as members with leadership voting rights until two weeks after the timetable for the contest has been set. This raises the prospect of a race among all candidates to get new supporters to sign up in order to back them, as happened in the 2015 and 2016 leadership contests that Corbyn won.

No candidate has formally declared, but Nandy, a former shadow cabinet minister for Miliband on the soft left, said on Sunday that she was “seriously thinking” about a run. Nandy said there was a “very, very hard road” to regain the trust of Labour voters in towns across the north of England, and she called on the party to move its headquarters and conference outside of big cities.

The Fabian Society, the socialist society and thinktank, warned that to win a majority at the next election Labour needs to gain 123 seats, almost twice as many as it required at the 2019 election.

It highlighted the scale of the challenge facing the new leader, saying that to secure the “winning post” marginal seat Labour now needs an electoral swing of 10.3 percentage points, almost three times more than the swing it needed to win the 2019 election.

It said 63% of the seats Labour needs to win are in the north, the Midlands and Wales, while 104 of the 123 seats Labour needs are in towns not cities.

Andrew Harrop, the Fabian Society’s general secretary, said: “The detailed numbers show that Thursday’s result was even worse than it appeared on the night. Our analysis shows that Labour will need to make huge strides in this parliament to have a hope of winning power even in 10 years’ time. A decisive change in direction is therefore needed.

“The party’s top priority must be to win support in towns and small cities in Wales, the north and the Midlands. There is no other route to winning back power. The party must choose its new leadership wisely, by asking who can earn the trust of potential voters in these areas, while not alienating existing supporters in big cities.”

After a weekend of reflection, Labour MPs and other figures have begun to give their verdicts on what went wrong. Burgon, who is considering running to be Long-Bailey’s deputy, admitted that the party’s position on holding another EU referendum “did fail” but he also blamed attacks on Corbyn by the rightwing media.

Q&A: How will the Labour party choose its next leader? Read more

Andy Burnham, who won 63% of the vote to become Greater Manchester’s first elected mayor in 2017, said he had been sidelined from the election campaign along with Labour’s other mayors and local council leaders.

“I wasn’t asked to do anything by the national party and I think I do have something to offer. I’m not offended by it, but I think it reflects the disconnect which has led us to this pretty dark place,” he said.

Burnham said the national party did not seem proud of policies that had not been dreamt up in the London HQ. “I would like them to be a bit prouder of what Labour figures outside of Westminster are doing to protect people from the worst of the Tory government,” he said, adding that initiatives such as his Bed Every Night scheme, which offers all rough sleepers in Greater Manchester a place to sleep indoors, were “utterly invisible” in the campaign.

Other MPs said it was necessary not to blame solely the leadership or Brexit but to acknowledge that the party’s hold over its northern heartlands had been weakening for some time.

Louise Haigh, a shadow minister, said: “We have to be honest about how we got here. The leadership was of course a factor, but so too were the years of bitter infighting and the party’s neglect, both perceived and real, of areas of the UK over many years.”

