The leader of Britain’s biggest trade union has vowed to defend Jeremy Corbyn against an attempted “coup” by Labour MPs, as the party’s divisions over Syria air strikes deepened.

Len McCluskey, the general secretary of Unite, warned that the entire union movement and the broader party membership would strongly “resist” pro-war Labour MPs seeking to force through a change of leadership.

“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,” he said.

Mr McCluskey’s bullish backing for Mr Corbyn came after the Labour leader told his increasingly rebellious Shadow Cabinet that he alone has the power to decide whether to order Labour MPs to oppose any British involvement in Syria’s civil war or let them vote according to their consciences.

Mr Corbyn’s unbending opposition to war could yet prevent British warplanes from attacking Isis targets in Syria, because David Cameron has said he will not seek Parliament’s approval to begin bombing unless he is sure of a majority.

The issue has split Labour MPs, with some threatening to vote with the Government even if Mr Corbyn tries to whip them to vote against.

Leading members of the Shadow Cabinet have pleaded with Mr Corbyn to defuse the row by giving them a free vote, but on 29 November he appeared ready to try to whip them into line, saying they should listen to the voices of party members who oppose going to war. “It’s the leader who decides,” he said.

This show of authority has infuriated old hands in the Labour Party who remember those years when Mr Corbyn voted against the party whips on hundreds of occasions. “This is not Jeremy Corbyn,” one highly placed source said. “He is one who has always believed in more internal democracy, not less. He has always believed in reducing the power of the party leader; now he’s being the most centralising leader in Labour Party history. This is a small cabal within the leader’s office.”

Len McCluskey (Getty)

But Mr McCluskey, whose Unite union is Labour’s biggest source of funds, writing for Huffington Post, said: “He [Corbyn] has been denounced for writing to MPs and party members making his views on Syria clear – as if his huge mandate, which included support for his long-standing anti-war record, had simply earned him the right to be seen but not heard.

“Yet at the same time members of the Shadow Cabinet are making their own pro-bombing views plain, either publicly or in off-the-record briefings. And 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 he warned: “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.”

On the other side of the argument there are Labour MPs who say they will vote for military action against Isis even if Jeremy Corbyn imposes a whip telling them to vote against. The latest to put her head over the parapet, Siobhain McDonagh, is a former government whip. “I would do the right thing, and I think the right thing is to support the Government in bombing Syria,” she told The Independent. “I wouldn’t do it lightly, because I’m the loyalest of the loyal.”

The former shadow cabinet member Chuka Umunna, the former cabinet minister Liam Byrne and the new MP Jess Phillips also indicated that they were prepared to defy the whip, if Mr Corbyn imposed it. Mr Umunna told Sky News: “On these issues you have to do what you believe is right.”

Labour’s Deputy Leader, Tom Watson, the shadow Foreign Secretary Hilary Benn, the shadow Justice Secretary Lord Falconer and the shadow Chancellor John McDonnell have all urged Mr Corbyn to allow a free vote.

That position is likely to be supported by a majority of the Shadow Cabinet when they meet on 30 November, but Mr Corbyn will come to the meeting armed with the feedback from an email he sent to party members canvassing their views, which had prompted 70,000 replies by early on 28 November. His supporters are also seeking to organise a Twitter storm to coincide with the start of the weekly meeting of Labour MPs. “The membership of the Labour Party must have a voice. Labour MPs need to listen to that voice,” he told the BBC’s Andrew Marr programme.

This open conflict between the leader and the Shadow Cabinet is so unusual that there is confusion about who has the authority to decide whether or not to whip Labour MPs. But members of the Shadow Cabinet believe that it is written into the party’s standing orders that the power rests with the Shadow Cabinet, not the leader alone.

The Defence Secretary Michael Fallon said he had been briefing Labour MPs on military action over the weekend but stressed the Government does “not yet” have a guaranteed majority to back air strikes.

The Defence Secretary rejected claims that bombing Isis-held cities could lead to a large number of civilian casualties as the terror group retreats into tunnels or uses the local population as human shields.

He claimed the RAF’s precision air strikes had not claimed a single civilian life during action taken against Isis in Iraq, and that Britain had “very strict rules of engagement”, and warned that the UK’s reputation would be damaged and the population less safe if action were not taken in Syria.

Pressed on the nature of the 70,000-strong force in Syria Mr Cameron referred to, Mr Fallon said: “We do know who they are and this is an independent joint intelligence committee assessment, it’s not ministers making this figure – it’s their assessment and it’s supported by academics.”

Jeremy Corbyn on The Andrew Marr Show (PA)

Mr McDonnell told Pienaar’s Politics on BBC Radio 5 Live that Mr Corbyn would take a decision on which way to whip in the morning, and make a recommendation to the shadow cabinet meeting.

“I want on these sort of issues an unwhipped vote, because they are above party politics,” he said. “I am hoping he’s taking my view into account the same as other party members.”

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