The evening of 12 October 2015 will live long in the memory of Labour MPs. All sides still speak with shock about the venomous exchanges that took place at Jeremy Corbyn’s second meeting with the parliamentary party. Veteran backbenchers described it as the most fractious gathering in 30 years. “A total fucking shambles,” was the verdict of the former cabinet minister Ben Bradshaw as he left.

Relations between Corbyn and his MPs were destined to be lukewarm at best. As few as 14 of them voted for him in the leadership election. But after an even-tempered conference in Brighton, it was hoped that the underlying tensions could be contained. A succession of events – Corbyn’s failure to be inducted into the Privy Council, the creation of a group called Momentum, the shadow chancellor John McDonnell’s reversal of support for the Conservatives’ so-called fiscal charter – brought them into the open.

MPs are prepared to accept political differences but are less tolerant of what they regard as the leader’s failure to fulfil the basic duties of leadership. Even allies despair at the confusion that has often characterised his opening weeks. “They’re not managing the party, they’re not managing the media,” one told me.

There was anger and surprise when McDonnell, without a meeting of either MPs or the shadow cabinet, announced that Labour would no longer support George Osborne’s fiscal charter. MPs were left asking why an avowedly anti-austerity leadership had backed the measure to begin with.

Few were satisfied with McDonnell’s explanation that global economic conditions and a visit to the now-closed Redcar steel plant had persuaded him to reverse his position. Rather, they suggest that he was unaware that the charter mandated an absolute budget surplus, instead of merely a current surplus (a claim he denied). Others say he failed to realise that, as a statutory instrument, the charter could not be amended to allow borrowing for investment.

If few MPs defend Corbyn’s leadership with confidence, even fewer align themselves with his most trenchant critics. The view of most is that Ian Austin, who told Corbyn to start acting like the leader of the opposition, rather than a “student union president”, and John Mann, who raged over the lack of policy direction, overreached themselves. “Once basic civility is abandoned in an organisation it’s a free-for-all,” a shadow minister said. Corbyn’s parliamentary allies are appealing for patience. Clive Lewis, the newly elected MP for Norwich South, whom some speak of as a future left-wing leadership candidate, told me: “There are going to be teething problems. The leader’s office is still being set up; John McDonnell changed his position from conference two weeks ago. I get all of that, but I don’t think the way that some people in that room behaved was warranted or justified. The way that Jeremy and John sat there quite stoically and respectfully, and took it, says a lot about them.”

The creation of Momentum is the cause of much of the unrest among MPs. The new group describes itself as a “grass-roots movement”, set up to harness the energy of Corbyn’s leadership campaign. But MPs, who were not briefed in advance of its launch on 8 October, fear it will become a vehicle for the deselection of sceptics.

Momentum's supporters cannot fully rebut this charge. Forthcoming boundary changes will force selection contests in some seats and activists can already initiate “trigger ballots” against incumbents. But Corbynites are seeking to reassure their colleagues. Katy Clark, one of Momentum’s six directors, told me: “The reality is, if you’re a good constituency MP, constituency Labour parties recognise that. I would say to anybody who’s worried about new people coming into the Labour Party: embrace it, work with the new members, engage them.”

The individual spoken of most darkly by MPs is Jon Lansman. As a veteran of the Bennite Campaign for Labour Party Democracy, Lansman, another director of Momentum, is an unashamed advocate of mandatory reselection. “When there are selections of an MP, I would like to see MPs who reflect the values of members of the party,” he said recently. “The fact is that Liz Kendall got 4 per cent of the votes in the leadership contest.”

Even left-wingers have been disturbed by his remarks. “Jon Lansman needs to wind his neck in and get back in his box. He’s doing a lot of damage,” a pro-Corbyn MP told me.

Frank Field, the chair of the Commons work and pensions select committee, told me that any MPs “picked off” through deselection should “cause a by-election immediately and stand as independent Labour candidates”. He said they “would probably win” and that “a whole pile” of colleagues would campaign for them.

All sides have identified next May, the month of London mayoral, Scottish, Welsh and 126 English local elections, as a defining early test of Corbyn’s leadership. Should Labour fall below expectations, he will come under intense pressure. Yet his weakness among MPs is more than counterbalanced by his strength among members. The party has nearly doubled in size since the general election, with 370,658 full members. To adapt Stalin on the pope: how many divisions do the anti-Corbynites have?

Many agree with the view of Ken Livingstone, who told me: “If MPs trigger another leadership ballot, Jeremy will be elected by an even bigger margin.” He added that the rules should be changed to allow anyone to stand for the leadership if they are “nominated and seconded by two MPs”, a move that would guarantee left-wing candidates a place in future races. But as an increasing number of Corbyn’s opponents acknowledge, no one should assume that there will soon be a vacancy.