When Jeremy Corbyn was elected as Labour leader in September 2015, most of his fellow MPs regarded his unexpected victory in the same way that you might think of a nasty cold: unpleasant, but ultimately something that would take care of itself. Optimists hoped that he could be dislodged midway through the parliament, perhaps in the summer of 2017, when little else would be going on. Pessimists expected him to lead Labour to cataclysmic defeat in 2020, which was then the due date for the next election. Both groups believed that when Corbyn stepped down, the institutions of the Labour Party would be essentially unchanged.

That calculation meant that informal conversations between some of Corbyn’s most committed Labour critics and the Liberal Democrats, including Nick Clegg and Norman Lamb, stalled quickly. Labour MPs were intensely loyal to the party brand, as one Lib Dem MP complained to me at the time: “It’s all ‘my grandfather was in this party, and his grandfather was in this party’. And really, what can you do about people who think like that?” Yet a bigger part of the reluctance to discuss a Labour split was that Corbynism was always meant to be a temporary proposition.

That calculation has changed. The 2017 general election, far from proving how electorally disastrous a left-wing platform and leader were, instead cemented Corbyn’s position. He secured 40 per cent of the vote and deprived Theresa May of her majority. Since then, he has secured his preferred candidate, Jennie Formby, as Labour’s general secretary, and the left will make up the majority of floor delegates at party conference, where rules and policies are decided.

All nine of the members’ representatives on the ruling National Executive Committee will have voted for him as leader in both 2015 and 2016. The only question is whether all nine of the original Momentum slate will be elected, or if the independent-minded Corbyn supporter Ann Black will take a seat from Peter Willsman, who was dropped by the grass-roots group after denying the extent of anti-Semitism in Labour. No candidates from centre-left factions such as Progress, or even the soft left, are in with a chance. “Our best case scenario,” one Labour MP sighs, “keeps getting worse. Two years ago, we were hoping to defeat Ann Black. Now we’re hoping that she might squeak on.”

Jeremy Corbyn’s undisputed dominance has therefore left disgruntled MPs with two choices. As one of their number puts it: “There’s Plan A, where you say, ‘We’re not going anywhere even if it takes 30 years.’ There is no red line, nothing that he could say or do that would mean you ever leave. Then there’s Plan B: you accept that he is not going anywhere, that the party has changed irrevocably into something else, and what do you do about that?”

In other words: is it time for Corbyn’s most trenchant critics to leave the party? Are we on the verge of a Labour split? To understand that, you need to comprehend how opposition to Corbyn has fractured into four groups: the Stay and Fighters; the Conscientious Objectors; the Brexit Firsters and the Policy Platformers.

One date haunts those considering a breakaway: 1983. When the Gang of Four – Roy Jenkins, David Owen, Bill Rodgers and Shirley Williams – broke away from Labour to form the Social Democratic Party two years earlier, they attracted widespread popular support. In the 1983 election, the SDP-Liberal Alliance won 7.8 million votes to Labour’s 8.5 million. But thanks to first-past-the-post, the new alliance won just 23 seats compared with Labour’s 209 (and 397 for the Conservatives).

The Gang of Four are therefore widely blamed on both the left and right of the Labour Party for splitting the vote and extending the Conservatives’ stay in office. Denis Healey, a darling of the Labour right, wrote in his memoirs that the SDP’s “most important effect was to delay the Labour Party’s recovery for nearly ten years, and to guarantee Mrs Thatcher two more terms in office”.

Today, the Labour leadership still fears that a party split could keep the Tories in power, and its outriders in the press regularly condemn the idea. However, there is another, more recent example on the minds of Corbynsceptic Labour MPs: Emmanuel Macron, who broke away from the Socialist Party in France. His new party, En Marche, carried him not only to the presidency but to a parliamentary majority, leaving the Socialists reduced to a fractured rump.

“I’m not philosophically opposed to a split,” one Labour MP told me recently. “What I am opposed to is splitting the left vote and letting the Tories in, and I can’t work out how you can know for sure that you aren’t risking that in advance.”

For most Labour MPs, though, tactical considerations don’t come into it. Leaving the party would be like losing a limb, or walking out on their families. Others worry about making ends meet if they walk away from politics. A third group is fuelled by long-standing antipathy to the Labour left – a hatred reinforced over long years of internal battle in local parties, student politics and trade unions. This “stay and fight” tendency have no intention of leaving Corbyn in possession of the party.

Among a small group of Labour MPs, that consensus is breaking down. Although they represent a minority of a minority – around a dozen of the most strident Corbynsceptics – there is a small group of Labour MPs who are planning in earnest to break away. The only questions are when and how.

So what has changed? As so often in politics, the personal element is as important as the ideological. Labour MPs from the party’s right have, for much of the Corbyn era, been part of a group on the instant messaging service WhatsApp called Birthday Club. There, they commiserate over the state of British politics, their casework load and their objections to the shadow cabinet. (The group was originally created to plan for a member’s birthday, hence the name.)

Most MPs venting via WhatsAppare not contemplating a breakaway and would not join one. Yet the conversations within them have helped dissident MPs bolster each other’s courage and discover what others are thinking. “It’s just like any other WhatsApp group,” one MP, who is opposed to any breakaway, says. “In that you can tell sometimes that people are forming other groups to talk about the group. They’ve stopped including me in talks about the future, because they know I would never leave [Labour].”

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Talks about a split are sufficiently advanced that they have moved from the digital to the physical realm, with an unlikely convenor: Tony Blair’s former chief-of-staff Jonathan Powell. He has organised several events and conversations between distressed Corbynsceptic MPs. (One well-placed source describes Powell’s events as a “holding pen” for Labourites who are contemplating a schism.)

Among Labour MPs, the question of who attends Jonathan Powell’s meetings is a topic of frequent speculation. The names that come up most frequently among their colleagues are Chuka Umunna, Liz Kendall, Chris Leslie, Gavin Shuker, Emma Reynolds, Alison McGovern and Wes Streeting.

Yet the truth is that Streeting and McGovern would never leave Labour, while former leadership candidate Kendall is still critical of the idea of a split, both in public and privately, according to several who have spoken to her.

Umunna, Kendall, Leslie and Shuker were all named in the Daily Express as attending a recent away day at the luxury Sussex bed-and-breakfast Fair Oak Farm, along with five other unnamed guests. The Express had to retract and apologise for naming Stephen Kinnock as one of their number, who was not invited and did not attend. He dubbed the story “fake news” and said “I've attended no such meetings. In fact I haven't been down to Sussex since our last Party conference in Brighton. Shame - it’s a lovely bit of the world!” The event was described as a plot against Corbyn or a discussion of a new party.

“The Express story was overwritten in many ways,” says one. “Not least because no one there, and no one with half a brain, thinks there is any prospect of getting rid of Corbyn. The conversation was about what you do about the fact that there is no prospect of getting rid of Corbyn.”

One of the first questions that arises whenever a split is discussed, though, is a very simple one: is there anything that unites Corbyn’s critics, except their criticism of the Labour leader? This worry preoccupies the Policy Platformer group. “If you look at the people who are seriously talking about splitting,” says one of the Fair Oak Farm attendees, “the problem is that they only agree on one thing: they don’t like Jeremy. I don’t like Jeremy either, but we have to have a positive policy.”

As a result, there will be more meetings like the one in Sussex and a greater effort to be “more courageous” in arguing explicitly for social democracy as a governing ideology within the party, according to one MP from the Stay and Fight faction. For others – the Brexit Firsters – the question of a new party is a distraction from “stopping or softening Brexit”. “We can worry about that [a new party] afterwards, one way or the other,” says one Sussex guest, who describes herself as “emotionally disconnected” from the Labour Party. “What matters now is Brexit.”

Then comes the next question: for whom would a new party seek to bid: disaffected Labour voters? Disaffected Conservatives? Remainers? All three? How would a new party treat the Liberal Democrats – perhaps through an alliance of the kind that the SDP and Macron (who formed a pact with an older French centrist party, the Democratic Movement) both struck?

Some disenchanted MPs feel these are all the wrong questions, and that their colleagues face a simple moral choice. Call this group the Conscientious Objectors. Their membership is fluid, with several MPs prepared to make a break if the party had pursued action against Margaret Hodge for calling Corbyn an “anti-Semite”.

“The essay question,” says another Fair Oak Farm participant, “is not as people are saying: where would it get votes? What would its policy platform be? Can we realign politics? It’s: if Jeremy Corbyn is immovable as Labour leader, which he is, can you morally go to the country and say that he should be prime minister? And if you can’t, everything else has to be secondary to that.” Among this group, the conversation has moved from whether they should leave to when – and how best to kick-start life after Labour. The likelihood is that exit would be a two-stage process: MPs would declare independence and only after that form a new grouping. (There is an awareness that any new party would need to seek an understanding with the Lib Dems to avoid fighting over the same policy turf, and perhaps plunder their resources and activist base.)

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One area where potential splitters know that Jeremy Corbyn is weak is Europe. The veteran Eurosceptic is aiming to appeal to Leave-voting seats in the north and Midlands without alienating an activist base that is overwhelmingly pro-European. As a result, there is a tendency to see anti-Brexit groups (including those calling for a “People’s Vote” on the final deal) as a front for anti-Corbyn sentiment.

On the Labour right, several pro-Europeans are wary of Open Britain, the successor organisation to the Remain campaign. They believe that its call for a People’s Vote is designed to provide a pretext for MPs such as Umunna to break away from Labour, rather than being a genuine strategy. “I am actually working flat-out to stop this [Brexit],” one Labour MP complained to me, “and I’m worried that colleagues aren’t really with me, so we’ll have a hard Brexit, a split on the left and a Conservative government for 20 years.”

Another believes that Umunna, who has been privately critical of the Labour leadership on a swathe of issues, is attempting the political equivalent of “suicide by cop: he is looking to be made into a martyr”. (Long-standing allies of Umunna strongly dispute this: one reason why the MP declined to serve in the shadow cabinet in 2015 was because he disagreed with Corbyn on Europe.)

Whatever the truth, it is undeniable that Brexit provides a strong disincentive to any split occurring before March 2019, as many sceptical MPs believe they can shift Labour policy to a more pro-Remain position before they leave. They are reluctant to give up on that prize. However, that calculation illuminates another problem for Labour’s dissident MPs: the timing, and indeed the scale, of their split is not entirely within their control. The party membership is part of the story, both in driving MPs to the exit and in limiting their ability to control the terms of a split. Corbynsceptic activists have been considerably more willing to walk out of the party than Corbynsceptic MPs – so the remaining members are more pro-Corbyn than ever.

Party memberships always churn, and within Labour the traffic has been one-way, thanks to the gradual shift to the left by successive leaders since Gordon Brown left office. Party activists are far more committed to Corbyn’s leadership than they were even in 2016, when six members in ten backed him over Owen Smith. One Labour MP recently updated his mailing list to be compliant with the General Data Protection Regulation. He discovered that while his local party membership was at a record high of 300, 200 members had left since Corbyn took over as leader. “People say: ‘Oh, we’d have no resources, no activists,’” he says. “But actually there is already a breakaway in the party membership, waiting to be led by someone. Why not us?”

While disenchanted MPs console themselves with the thought of activists waiting to embrace their new party, in the present that churn is causing them problems. The former shadow chancellor Chris Leslie, another MP widely believed to be contemplating a schism, has already seen pro-Corbyn members win control of all but one position in his local party in Nottingham East. He is known to be the number one deselection target in the east Midlands; many believe he has nothing to lose by leaving Labour. “In the end, it will be the membership who decides if there is a split,” says one Corbynsceptic MP. “Do they want to maintain a social democratic presence within the party and the PLP? Or do they want a far more narrow party?”

The Labour Party’s current rules are designed to protect incumbent Labour MPs. But the expectation is that they will soon change, passing the party conference floor comfortably thanks to Momentum’s hegemony among the grass roots and the support of several of the unions.

Even without rule changes, the expectation is that several Labour MPs will face deselection, with the chances particularly high on Merseyside, where the Corbynite left is well organised and buttressed by former members of the Militant Tendency. Louise Ellman, Luciana Berger, Stephen Twigg and Frank Field come from different Labour traditions, but they are all under threat.

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This is where we are, then, as Jeremy Corbyn concludes a difficult summer and prepares for another rapturous reception at Labour conference and Momentum’s sister festival, The World Transformed. The Stay and Fighters are hunkering down; the Policy Platformers are trying to work out how to unite the disparate strands of anti-Corbyn feeling; the Brexit Firsters are waiting to see how leaving the EU works out before they do anything else; and the Conscientious Objectors are wondering how much longer they can hold on.

All these groups share the same belief: that they are best placed to oppose Corbyn. But what if they are wrong? Over the last few months, a little-known entrepreneur called Simon Franks has been touring Britain to drum up support for a new party. If he is successful, that would change the splitters’ calculations profoundly; their new grouping would not be the only game in town. Franks is in many ways the model of a departed Labour activist: the 46-year-old joined the party while Blair was leader and left when Corbyn was elected. The millionaire founder of LoveFilm was one of the vanishingly small group of businesspeople willing to be publicly identified with Labour under Ed Miliband, and he served as one of Ed Balls’s business advisers. (Most Labour MPs who met him at the time found him conceited and arrogant, I am told.)

Franks believes that a new party can succeed – quickly – without waiting for support from existing MPs. In fact, some of his allies think that their chances of success are higher without, because distrust of the established parties is so high. His new grouping, United for Change, has already attracted financial support, though those involved concede that they will need to “swallow” not only the Lib Dems but any new grouping that emerges out of the Labour Party as well.

While the new group is well-funded, money may not be enough. “I know what I would do if you gave us £50m,” one senior Liberal Democrat told me. “But as for starting a new party, do you start with premises? Buying [data on potential voters] from the supermarkets? Polling? Digital infrastructure? There’s just so much to do.”

One Labour MP who worked with Franks under Miliband is dismissive: “He’s one of those businessmen who thinks he can just walk in and buy his way to political success. Actually they’re no better at politics than we would be at running a multinational and I think he’ll get a tough lesson.”

The task ahead is not that much easier for veteran politicians, however. But for one Labour MP, it comes back to a matter of conscience. “Once you work through the stages of grief: denial, bargaining, anger, and there’s a lot of those about in the PLP at the moment, you’re left with acceptance,” says the Fair Oak Farm guest. “Acceptance that you can’t remain in the Labour Party, no matter how difficult the circumstances of leaving are. And actually, once you get there, it’s quite freeing.”

Read Stephen Bush on who is likely to lead a Labour Party split here.

Corrections: The article originally stated that Lucy Powell intended to post her message about Angela Rayner and Tulip Siddiq, in which she criticised them and the state of the Labour Party, in a WhatsApp group called Birthday Club. (The message was instead sent to members of the women's PLP.) Powell is not a member of Birthday Club, and the accidental message was not intended for that group. We've updated the section on Fair Oak Farm to take account of the Express' apology to Stephen Kinnock, which was issued after we went to press.