Jeremy Corbyn is heading for a showdown with his shadow cabinet over his opposition to military action in Syria, as the boss of Britain’s biggest union warned that MPs who continue to show him a lack of respect are writing their political obituaries.

The Labour leader is facing a rebellion among his senior colleagues, with the shadow foreign secretary Hilary Benn, the deputy leader Tom Watson, the shadow lord chancellorCharles Falconer and the shadow education secretary Lucy Powell minded to support extending airstrikes against Islamic State.

They will gather for a crucial shadow cabinet meeting on Monday, at which it will be settled whether Labour can collectively come to a decision on military action or whether they will have to hold a free vote allowing MPs to go with their conscience.

If Corbyn decides to whip his party against military action, it becomes more likely that Downing Street will not proceed with a vote as Cameron has said he wants a clear majority of the House of Commons in favour of airstrikes.

The Labour leader asserted his authority over the party on Sunday, reminding MPs of his large mandate and making clear that he alone would decide whether to whip them to vote against extending airstrikes on Isis.

Cameron is likely to hold a vote on extending the bombing of Isis to Syria on Wednesday. He has the vast majority of Tory MPs behind him, but foreign secretary Philip Hammond and defence secretary Michael Fallon have been phoning Labour MPs urging them to swing behind the government position.

Facebook Twitter Pinterest Hilary Benn is one of Corbyn’s most high-ranking opponents on Syria. Photograph: David Gadd/Sportsphoto Ltd/Allstar

Some shadow cabinet ministers believe they can overrule Corbyn and force him to hold a free vote. But Corbyn retains the option of simply sacking those who try to defy him and replacing them with colleagues who share his view.

One senior Labour source, who is not in Corbyn’s camp, said Labour MPs seemed to be getting more wary of backing military action over the weekend for fear of “marking their card” and getting singled out as targets for possible deselection by activists.

On Sunday night, discussions about whether Corbyn would allow MPs to vote with their conscience were still continuing, but there is a possibility of compromise on how a free vote is carried out or the level of whipping imposed. Negotiations involve who would be allowed to speak from the frontbench in the debates.

After a tumultuous week in the party, Corbyn warned plotters that he was “not going anywhere”.

At least three Labour backbenchers – John Spellar, Fiona Mactaggart and Simon Danczuk – have suggested it would be better for Corbyn to step down and some senior party figures have taken legal advice about whether they could exclude him from future ballot papers.

In an acknowledgement there may be Labour figures trying to plot a coup against him, Corbyn told the BBC’s Andrew Marr Show: “I feel there are some people who haven’t quite got used to the idea the party is in a different place.”

Len McCluskey, the general secretary of Unite, went even further than Corbyn in accusing some within the party of using the issue of Syria as “the thin edge to stage a coup against Jeremy Corbyn, Labour’s (overwhelmingly) elected leader”.

Writing in the Huffington Post, he said: “Backbench MPs are even calling on him to quit for having the temerity to maintain his values and principles ... the thought that some Labour MPs might be prepared to play intra-party politics over an issue such as this will sicken all decent people. And they are playing with fire. Any attempt to force Labour’s leader out through a Westminster Palace coup will be resisted all the way by Unite and, I believe, most party members and affiliated unions.

“I do not know whether there will be a free vote on Syria or not. However, that is a decision for Jeremy to make. But we cannot have a free-for-all party. If those Westminster bubble-dwellers who hanker back to the politics of the past cannot show the elected leader – and those who voted for him – more respect, then they are writing their own political obituaries.”



In a bid to persuade Labour MPs to swing behind his view, Corbyn has also consulted members about their views on whether Cameron has made the case for bombing the Isis stronghold of Raqqa and received 70,000 responses through an online form.



Facebook Twitter Pinterest Len McCluskey highlighted the size of Corbyn’s mandate as a reason to back him. Photograph: Christopher Thomond/The Guardian

It is understood that sampling has shown about 70% of responses are against the extension of war against Isis into Syria. Momentum, the grassroots group of Corbyn supporters, has separately been urging people to lobby their MPs against supporting airstrikes.

Corbyn is also seeking the support of party’s ruling national executive committee in an attempt to persuade MPs to come round to his view. “The Labour membership must have a voice,” he said. “Labour MPs need to listen to that voice and try to understand where they are coming from.”

Amid signs Labour MPs are increasingly nervous about backing airstrikes in defiance of the leadership, Watson has asked for further clarification from Downing Street on the government’s claims that about 70,000 moderate Syrian rebels are being prepared to fight Isis on the ground.

The deputy leader has been trying to broker a deal with Corbyn to allow a free vote, as it is possible that key shadow cabinet ministers would have to resign if the leader insists they vote against airstrikes. John McDonnell, the shadow chancellor, is in favour of a free vote across all parties as a matter of conscience.

McDonnell said people should not see Labour’s discussions about Syria as divisions but a healthy and democratic sign of the new politics that Corbyn has brought to the party.

Many Labour MPs have now made clear their positions either for or against military action, making the idea of a whipped vote increasingly difficult.

Lord Falconer became another senior member of the shadow cabinet to confirm he was minded to support the government’s proposals. On the BBC’s Sunday Politics, he refused to comment on whether he would step down if the shadow cabinet was forced to take a collective position against military action but said he hoped there would be no need for resignations.

Chuka Umunna, the former shadow business secretary, who withdrew from the leadership campaign early in the contest, said he would vote on his conscience regardless of what the leadership decides and is minded to vote in favour of the government’s plans.