When Frank Field went to inform Labour’s chief whip, Nick Brown, that he was resigning as a Labour MP, the other man was suspicious. Mr Brown, a veteran of many a political plot, demanded of Mr Field whether he had been conspiring with other Labour MPs who were also on the brink of quitting. Was his declaration that he would sit in parliament as an independent, after 40 years as a Labour MP and nearly 60 as a member of the party, the first move of a deeper conspiracy? No, responded the MP for Birkenhead. He told the chief whip: “You are the first Labour MP I have spoken to about it.”

Having made my own inquiries, I believe this to be entirely true: Mr Field acted alone. Indeed, some other Labour MPs who are on the verge of leaving can be heard complaining that it would have been helpful if he’d given them a bit of advance notice. This was not a conspiracy – it was an act of utter despair.

We need to take care discussing the meaning of his case because he has always been an individualist who resisted easy labelling. He was a Labour MP who wasn’t scared to say that he found things to admire about Margaret Thatcher and also a man fiercely committed to fighting poverty. He led the Labour backbench revolt against Gordon Brown when he made tax changes that disadvantaged the poor. He is in that small minority of Labour MPs who support Brexit. This hasn’t helped his cause in acrimonious disputes with members of his local party. It also makes him very unalike most of the other Labour MPs who are contemplating resignation. Jeremy Corbyn’s reluctance to robustly oppose Brexit is one of their many quarrels with the Labour leader.

The MP for Birkenhead is a septuagenarian who doesn’t flinch from a scrap, as Philip Green discovered when Mr Field used his select committee chair to go after the retail tycoon over the BHS pensions. He fought his first battles with the hard left in the 1980s, but was among the Labour MPs who facilitated Jeremy Corbyn’s rise to the leadership by signing his nomination papers when he was short of the numbers needed to get into the race. Whenever I have reminded him of this, Mr Field has replied that he did so because he thought Mr Corbyn’s wing of the party deserved a voice in the contest to succeed Ed Miliband. Politics often throws up ironies and this one is particularly pungent. By giving a helping hand to Corbynism in the name of pluralism, Mr Field assisted in the creation of a Labour party that has become so toxified with intolerance that he feels he can no longer represent it in parliament.

Forecasts of how many might go and when it would be most likely to happen vary dramatically

A man who has often walked alone did the same when he walked out. His act was not intended as the opening gambit of a larger project for a breakaway from the Labour party, but it will nevertheless complicate the calibrations of both those contemplating that move and those who want to thwart it. Predictably, a hit squad of Corbynista propagandists has been mobilised to pelt Mr Field with abuse and try to trash his many decades of service in parliament and to his party. One of the things this tells us is that the Labour leadership is more worried about the impact of his resignation than they will admit. The sharper people at the top of the party will understand the reputational damage done when an MP of such longstanding quits the whip because the party “has become a force of antisemitism in British politics” under Mr Corbyn, who “is doing nothing substantive to address this erosion of our core values”.

The other central complaint of his resignation letter is about “a culture of intolerance, nastiness and intimidation” in local parties that the leadership “has allowed to grow unchecked”. This will resonate with both Labour MPs and the party’s remaining non-Corbynite members. It will likely have more moderate voters asking themselves: do we want to give a party that has turned so nasty any power over our lives?

For Jeremy Corbyn and his inner circle, the most disturbing question is the one asked by the chief whip: is Mr Field blazing a trail that other despairing Labour MPs will shortly follow? Is this a harbinger of more Labour MPs quitting, either to become independents or to organise a breakaway party of the centre-left? Corbynite strategists always maintain that they have no fears that a new party could beat them at the ballot box; what worries them is that it could attract enough support to deprive Labour of power by splitting its vote to the advantage of the Tories.

Opportunity knocks for a new party. But will anybody dare open the door? | Andrew Rawnsley Read more

The prospect of a breakaway is also feared by another Labour grouping that is represented in both parliament and the membership. These are people who hate what is happening to their party, but who will never abandon it and find it treacherous to even think about that. “I will be Labour until the day I die” is a refrain commonly heard from them. Some of this tendency acknowledge the moral dilemmas of campaigning for Jeremy Corbyn to become prime minister when they think he is unfit for the office, but they will nevertheless not quit.

This group hates the Corbynistas and the potential splitters in about equal measure. They are fearful of a breakaway because it would leave them a lessened force within Labour as well as jeopardising the party’s and their personal electoral prospects. They were given voice when Tom Watson, the deputy leader, described Mr Field’s resignation as “a major wake-up call” that reflected “both the deep divisions in the party and the sense of drift engulfing us”.

Then there are those Labour MPs who are contemplating resignation. They will tell you that quitting before the next election is the only honourable thing to do because it has become morally unsustainable for them to urge support for the party. For them, the Field resignation is uncomfortable because it pricks them to search their own souls. In the words of one of their number: “It is twanging consciences about why we haven’t gone yet and has people like me asking, why am I still in this disgusting organisation? It’s definitely not the bloody Labour party any more.”

Forecasts of how many might go and when it would be most likely to happen vary dramatically. It rather depends on who you are asking. What’s not in doubt, as I’ve reported to you before, is that a breakaway has been under deeply serious consideration for some time. One of the challenges for this group over the summer of Labour ferment has been persuading some of their number to wait rather than resign immediately. Mike Gapes, the Labour MP for the London seat of Ilford South, has said publicly that he is “agonising daily” about whether to quit over antisemitism. One of his colleagues says: “A lot of the summer has been about holding the gang together and stopping people splitting off, one by one. We’re all trying to stay in formation until the right moment.”

The more cautious potential breakaways often want to wait and see whether the Corbynistas are going to make an aggressive attempt to terminate their political careers by deselecting them as parliamentary candidates. Bolder breakaways say they need to act sooner rather than later to avoid the accusation that they are jumping only because they were going to be pushed.

The timing and size of a breakaway is not in anyone’s capacity to entirely control, not those trying to choreograph one and not those who want to prevent or minimise a split. Mike Gapes is not the only Labour MP agonising over what to do. Long loyalty to and love of the party are in contention with loathing of what is happening to it. Decisions about whether or not to quit will be profoundly personal ones and the result of many long, dark nights of tortured deliberation.

Brexit is another complication in everyone’s calculations. A lot of the potential breakaways think they have a duty and responsibility to stay within Labour until the party and parliamentary struggles over Brexit are resolved. The chance to soften or stop it is the argument I hear most often hear from likely breakaways when they are explaining why they haven’t gone already. “Brexit is the only thing keeping me in,” says one MP of this type. That reason for not quitting will have disappeared once Britain has departed the EU, which is scheduled to happen at the end of next March.

Frank Field acted alone, but I doubt he will be alone for all that long.

• Andrew Rawnsley is an Observer columnist