Labour leadership contests are normally traumatic events held in very unhappy times for the party. The election of Ed Miliband in 2010 followed a general election defeat that ended 13 years of Labour government. Five years later Jeremy Corbyn was installed after Miliband failed to return the party to power. Then in the summer of 2016 a revolt against Corbyn by his own MPs sparked another contest, which saw the incumbent win again and take revenge on the mutinous parliamentary party.

Inevitably these contests cause internal ruptures as rival candidates set out opposing visions and their supporters divide into camps. The 2010 campaign had its tragic aspects too, as the Miliband brothers, Ed and David, fell out while fighting each other for the right to succeed Gordon Brown.

But the race to take over from Corbyn, following Labour’s worst general election result since 1935, will begin next month amid levels of anger, recrimination and bitterness that few in the party can remember.

“The actual aftermath is proving to be even worse than the election loss itself and that was truly awful,” was the way one Labour MP who retained her seat described the atmosphere last week.

Former MPs and candidates who lost are furious with the leader and his advisers. So too are party workers, many of whom will now lose their jobs, while officials in high places in Corbyn’s office keep their high-salaried posts. At the same time hundreds of thousands of members, activists and supporters – buoyed by optimism – feel suddenly let down.

One party official, who is now looking elsewhere for work, expressed her anger at Corbyn and his team. “They are the architects of this defeat in every way,” she said. “The management of the entire campaign was atrocious. We piled resources into the wrong seats – ones we could never win – and failed to defend ones that were obviously under threat. Why should they stay and we be shown the door when they brought this on us?”

The way the Unite union, headed by Corbyn ally Len McCluskey, was able to install its own candidates late on into key seats has bred further resentment, adding to the sense that a hard-left clique around the leader held sway over all the key decisions.

The handling of the aftermath of defeat has made matters worse. Defeated candidates and MPs were not contacted by Corbyn or the party high command until late last week and even then all they received were identical impersonal letters, though some had short handwritten sentences written at the end by the leader. “It is like they said just print off 60 of those [the number of Labour MPs who lost their seats] and send them out would you?,” said one of the recipients. James Frith, who lost Bury North, was unimpressed by a note in Corbyn’s hand at the bottom of his circular letter. “So sorry at result and tks for all your work. Jeremy” was all the leader could muster.

The key question now is what effect all the bad feeling around the leadership will have on the succession battle. Rebecca Long-Bailey, shadow business secretary, is seen by some as the early favourite on the basis that she will have the Corbyn-dominated party machinery – the unions and the national executive committee (NEC) – behind her. Long-Bailey has long been touted by shadow chancellor John McDonnell as the one to carry the torch for the left after Corbyn. But with the atmosphere so toxic, and with Corbyn having proved so unpopular on doorsteps, is it really an advantage to be viewed as the “continuity Corbyn” candidate? Some of Long-Bailey’s friends say she now needs to strike out on her own. One MP who knows her views well says: “Becky is so ‘not Jeremy’. But the issue is can she break free of being their puppet? She knows what she needs to do and she needs to be her own person.”

Last week there were a series of difficult meetings about how to organise a Long-Bailey campaign. McDonnell is said to have walked out of one after it was suggested he should take no further part, though he has now agreed of his own accord not to be involved. “There is lots of left-on-left warfare going [on] over how keep control over policy and the machinery,” said one official. Disagreements between those around Corbyn about how to safeguard some sort of legacy seem to be holding up a Long-Bailey launch. One senior figure said: “I do find it strange that Rebecca has not tweeted once since the election. All the other candidates are stealing a bit of a march. I think it speaks to a lot of tension among her people.” There are signs, too, of division in the previously solidly pro-Corbyn grassroots organisation Momentum. On Friday, Laura Parker resigned as its national coordinator.

So far, only two candidates have announced officially that they will stand: Emily Thornberry and Clive Lewis. Keir Starmer is preparing to do so early in the new year under the slogan “Another Future is Possible”, making clear he will represent anything but continuity. Long-Bailey, Lisa Nandy, Jess Phillips, Yvette Cooper and David Lammy are considering when and whether to follow.

The bloodletting is under way and the bad feeling runs very deep. Declared and undeclared candidates want to break free and establish their own identities but fear that the left and the unions can still kill their campaigns at birth if they are too critical of the recent past. Corbynism has failed to win two elections but it still holds control over the levers of power.

One former minister who survived on election night said the candidate who would ultimately win would be the one who told the truth. “Someone needs to be honest and say we got it wrong, say to the members you got it wrong. We have probably been a bit cowed into thinking that the left and Momentum will control everything in this contest just because we are now used to them doing so. But this defeat is so colossal that I think it will have shaken the kaleidoscope. The person we need is the person who can speak that truth. If not, we are finished.”