As Big Ben strikes six on Monday evening, mainstream Labour MPs will congregate in the House of Commons’ wood-pannelled Committee Room 14 to launch an early fightback against what many fear is an inevitable victory for Jeremy Corbyn.

The Observer has learned that the chairman of the parliamentary party, John Cryer, has agreed to hold a debate among Labour MPs about the return of shadow cabinet elections, a move seen by many as a pivotal part of a plan to disrupt a swift “Corbynista” takeover of the party machine. A vote to bring the elections back can’t be held that evening because of various complicating rules, but it is likely be staged the following Monday – just 48 hours after the new leader’s election. Worries that party conference would subsequently need to endorse the MPs’ decision have been assuaged by officials.

After an extraordinary summer during which conspiring has had to be done remotely, the end of recess tomorrow will mark the start of feverish activity at the Palace of Westminster. One frontbencher said of tomorrow’s meeting: “There will be foment, torment and some will be in clover.”

The election of shadow cabinet ministers was ended by Ed Miliband in 2011 and, in recent weeks, Corbyn has spoken of appointing his own team. But despite some holding the view it might be best to let the Corbyn phenomenon come to its natural conclusion, (“a car crash”, is the consensus), Monday’s debate will be about an insistence that the parliamentary party, where Corbyn support is wafer-light, will not be going away.

Facebook Twitter Pinterest John Harris charts the rise of Jeremy Corbyn and his effect on politics, and finds a political world where everything is suddenly turned upside down

Barry Sheerman, the MP for Huddersfield who approached Cryer over holding the debate, said: “I have been re-reading the manifesto which I solemnly put before the electorate. We may have lost the election, but there are lots of interesting things in there, and it was the basis on which people elected us. I don’t know if a new leader can tear up that agreement with their constituents.”

Graham Stringer, the MP for Blackley and Broughton, added: “As Jeremy has in the past, I have always thought the shadow cabinet should be elected.” Simon Danczuk, the MP for Rochdale, is understood to have already looked with colleagues at how a slate of moderate MPs could be compiled to fill key posts.

The logic behind MPs laying the foundations of their resistance to Corbyn now and in this form is buried in the arcane governance of the Labour party. If the MP for Islington North held total power of patronage over the shadow cabinet, some fear he would pack it with leftwing allies and then use his right to appoint three of those shadow cabinet ministers on to Labour’s governing body, the National Executive Committee.

With control over the 33-strong NEC, which is finely balanced between the wings of the party, the theory goes that Corbyn would be able to rush through a series of radical changes at the annual party conference held two weeks after his election, with the effect of undermining the influence of MPs who do not share the new leader’s leftwing aspirations. While still holding hope of an Yvette Cooper victory, John Spellar MP, warned: “They won’t be able to help themselves.”

Documents produced by the Bennite Campaign for Labour Party Democracy – whose spokesman, Jon Lansman, is a key figure in the Corbyn campaign team – possibly offer an insight into the moves that would be made, including making it easier for local constituency parties to deselect their MPs and taking power from moderate MPs, giving it to Corbyn-enthused members.

An explainer on one motion ready-prepared for conference notes: “The procedure would reform the policy-making process to (i) provide for a rolling programme based on amendments from party units, giving grassroots party individuals and affiliated members direct input into policy making; (ii) enable party conference to make the final decisions on policy.”

Or, as one Corbyn supporter and Unite member, Nick Long, wrote in a rather more blunt letter to his local paper in Lewisham: “If Corbyn wins the leadership race and other progressive MPs join him, a battle royal will ensue to wrest control of the Labour party out of the hands of the Blairite and Progress elite and the adoption of an anti-austerity agenda. A long struggle will be needed to oust these rightwingers and careerists.”

The Corbyn team deny aspirations for such blood-letting but they are certainly looking at how to convert those who have paid £3 to vote in the Labour leadership election into full members, with voting rights. It is believed that the vast majority of those 120,000 supporters, who have swelled the Labour electorate to more than 550,000, share Corbyn’s policy aspirations. In a sign of his determination to hard-wire the new supporters into the party machine, last week Corbyn told one of his rallies: “This level of participation is unprecedented and I am delighted by that. It will change the Labour party.

“I think it will make it a much more democratic organisation; it will make it much more participatory. My determination – it’s not about me, it’s about us, there’s a whole lot of us – is that we have a much stronger grassroots democracy, so that ideas come up, rather than decisions are made at the top and are handed down like papal encyclicals.”

Facebook Twitter Pinterest Jeremy Corbyn, the frontrunner in the Labour party’s leadership contest, says that he would like to see policy made by Labour members and not the leader

All very reasonable, it might seem. “But there is an ulterior motive,” said Luke Akehurst, a Labour councillor and former NEC member. “The analogy an MP gave me was that these people are moving through the party like Isis in their jeeps in Iraq. They need to push on, take over, before they lose the momentum.” Perhaps not the most tasteful of analogies, but these are traumatic times for Labour MPs who have spent a lifetime opposing the hard left. One shadow minister said he had written his resignation letter and said he planned to “act like a Lib Dem for the next five years”.

“I will stay in my constituency, make myself useful, pick up dog shit and stay away from Westminster. What is the point of going there?” he said.

However others, with no less frustration have been examining the reasons behind the leftwing candidate’s rise and how they should respond.

Among those reading the runes are members of Miliband’s former office who are furious that the new leadership election rules – a one-person one-vote system that allowed new people a vote for £3 – were largely ignored as a potential factor in the contest by candidates other than Corbyn. One former Miliband aide said that of the candidates’ websites at the start of the contest only Corbyn’s contained a link to the site that would allow people to become “supporters”.

But the conservatism of some in the party had become apparent, the former aide claimed, even before the general election defeat. Two of Miliband’s inner circle – his director of strategy Tom Baldwin, and speechwriter Marc Stears – had suggested that the party seek out £3 supporters before 7 May in an attempt to engage people with the Labour party.

The membership had already changed hugely by 2015 compared with 2010, possibly in favour of the left. Only 35% of the 185,000-strong membership in 2010 was retained, and by 2015 new blood had lifted it back to around 200,000.

“Lots of Blairites left in a sulk because David Miliband wasn’t leader and it is generally the case that those that then joined are sympathetic to the leader,” said the source. But for all the party’s talk of radicalism, the idea of a membership push by MPs and officials to ignite interest and build a broader church was dismissed as “hippy nonsense” by more conservative voices around the leader.

Indeed, the Observer has learned that such was the lack of appetite for exploiting the rule changes that it was decided to ignore them when it came to the Scottish leadership election in December 2014. “No one noticed,” the source said. “But that election should have been done on the one-man one-vote basis with £3 supporters allowed to be involved. It was decided that it would be an embarrassment, with only about 10,000 members voting. And so the old system was used.There was considerable institutional conservatism, and the unions didn’t want to engage with it. And, other than Corbyn, the candidates in this contest have also been terrified of new people.”

Responding to the complaints about the rules by some senior figures, such as Michael Dugher MP, who is managing Andy Burnham’s campaign, the source added: “My strong advice is that if in an election you don’t have enough votes, you should go and find some more.”

It is advice that some are now belatedly ready to take. One frontbench spokesman said that with shadow cabinet positions came the chance to make speeches at conference and catch the public’s eye, and that an opportunity to build excitement around a candidate of the centre-left will not be lost next time. “There is a good chance that if Corbyn has a bad conference speech and three or four bad prime minister’s questions, the support around him will drop away,” the shadow minister said. “I am talking about the unions really, who will have strength in numbers on the NEC and finance the whole thing.

“Dave Prentis at Unison and Paul Kenny at GMB might put on leftwing clothing when it suits, but will they want to stick with a leader who is a laughing stock and can’t attain the rights for their members by winning an election? It might be over in six to 18 months. I don’t think the left have someone with Jeremy’s personal warmth who could stand.

“So it then needs someone in the mainstream to shine at conference and start enthusing people. Some have said Dan Jarvis could be the one. But he needs to be able to make a barnstorming speech. Tony Blair and Gordon Brown could do it. Can he?”

The MPs gathering on Monday night are looking to provide a platform for Corbyn’s successor – but who can perform on it?

Decision day: How the party’s new leader will be revealed

■ The special leadership conference at the Queen Elizabeth Conference Centre in Westminster starts at 10am on Saturday, when the four candidates arrive to be ushered into the “candidates’ holding room”. At 10.30 they and their campaign managers will learn the results but be sworn to secrecy. At 11am, Iain McNicol, the party’s general secretary, will open the conference and the candidates will be invited to take their seats at the front of the audience.

■ Before the leadership result is announced, the matter of the deputy leadership needs to be dealt with. The chairman of the national executive committee, Jim Kennedy, will draw the audience’s attention to a big screen where the results of the alternative vote poll will be shown, round-by-round, so that members can see who received the most and least first preferences, and how the second preferences of voters accumulated to provide a winner of the contest. The winner will be announced and he or she will make a short speech.

■ At around 11.30am Kennedy will return to the lectern to do it all over again for the leadership contest. After the winner is announced, at 11.45, Ed Miliband’s replacement as Labour leader will make his or her first speech as leader of the opposition or, as some have described it, the worst job in politics. It will be the end of a tortuous morning, the start of a gruelling few days and the launchpad for a daunting five years. A blizzard of media interviews will then begin, before a 3pm meeting with senior management of the party.

■ Sunday morning will bring an interview with Andrew Marr on his BBC TV programme. Activities on the Monday will range from “cake and fizz” with party staff at lunchtime to a private meeting with the general secretary and a first meeting of the parliamentary Labour party and the shadow cabinet. Should Jeremy Corbyn be chosen, he will also attend an uncomfortable meeting with Alan Johnson, who is leading Labour’s pro-EU campaign. Corbyn is the least Europhile of the candidates.

■ The new Labour leader has also been pencilled in on the Tuesday to attend a commemoration of the 75th anniversary of the Battle of Britain. If Corbyn wins, does he wear his vest and Breton cap?

Daniel Boffey