Free vote talked up by senior Labour figures to get through Syria row

The Prime Minister is believed to be drawing up the parliamentary motion this weekend. He said he believed he had won over some wavering MPs and urged them to "vote on the basis of the arguments".

Philip Hammond, the Foreign Secretary, claimed there was now growing "momentum" among MPs in support of airstrikes.

As Jeremy Corbyn faced the first calls from colleagues for him to step down, warnings were made that if the party leader sought to force the shadow cabinet into opposing airstrikes, then he would face a string of resignations.

Most of its 28 members spoke in favour of airstrikes. It is thought only four shadow cabinet members support Mr Corbyn’s opposition to airstrikes.

It was claimed the shadow cabinet, which met on Thursday after Mr Cameron had set out his case for extending airstrikes, had agreed to mull over the issues this weekend, say nothing and reconvene on Monday.

But within two hours Mr Corbyn’s letter to Labour MPs had been released. In it, he explained the reasons why he could not support further military action; a “convincing case” had not been made.

But this “lit the blue touch-paper” for many frontbenchers, who felt it was a betrayal of an agreed position on keeping silent and a clear attempt to mobilise the grassroots to pressure their MPs to vote against military action. People were “incandescent” with rage, said one party source.

Labour backbencher John Spellar, a member of the Commons Defence Committee, said Mr Corbyn's behaviour had been "unacceptable" and he urged shadow cabinet members considering resignation to stand their ground, saying: "If anyone should resign after this incident, it should be Jeremy Corbyn."

His colleague Fiona Mactaggart, another former minister, said Mr Corbyn's leadership was "weak" and "unsustainable" and his stepping down would be a “sensible strategy”; although she was also unconvinced about the case for bombing Syria.

Labour leftwinger Paul Flynn, another opponent of airstrikes, described the atmosphere within the party as “poisonous”.

The Welsh MP revealed he had told his leader he would have to resign if he became a "liability" to the party and that Mr Corbyn "understands that".

After John McDonnell, the Shadow Chancellor, urged senior colleagues to “calm down” and “not mistake democracy for division," a number of them signalled a free vote - allowing frontbenchers and backbenchers to vote with their consciences - was the best, and perhaps only, way forward.

Tom Watson, the deputy leader, who is in favour of airstrikes, strongly hinted that a free vote would take place.

Asked if he would resign if there were not a free vote, the Midlands MP said: "No, of course not. I'm the deputy leader of the party with a mandate. But I don't think that situation is going to occur."

Hilary Benn, the shadow Foreign Secretary, who also backs airstrikes, stressed how Labour MPs might "end up" being given a free vote to avoid further rows.

Meantime, Diane Abbott, the shadow International Development secretary and a key ally of Mr Corbyn’s, argued that his opposition to airstrikes chimed with the majority of Labour members.

She called on her shadow cabinet colleagues to support their leader, saying: "Jeremy appoints the shadow cabinet - not the other way round. You cannot have a shadow cabinet voting down the leader of the Labour Party, who has just been elected with the biggest mandate in history."

Mr Corbyn, who cancelled a trip to Oldham to campaign ahead of next week’s by-election to deal with the fall-out to his letter, will reconvene the shadow cabinet on Monday. Later in the evening, a potentially dramatic meeting of the Parliamentary Labour Party will take place.

One Labour MP noted: “I shudder to think what will happen there.”