Jeremy Corbyn has thrown down the gauntlet to his Shadow Cabinet and Labour MPs after declaring he will not back David Cameron's call to launch airstrikes in Syria.
Triggering what looks like a huge power struggle at the top of the Labour Party and the prospect of resignations, Mr Corbyn has written a letter to Labour MPs and peers saying he “cannot support” the proposal outlined today for bombing to get rid of ISIL.
His position is at odds with many of his Shadow Cabinet and comes on the same day as his Shadow Foreign Secretary Hilary Benn said there was a “compelling case” to join the air raids.
Members of the Shadow Cabinet told HuffPost UK how the letter was not mentioned at a meeting just a few hours before, where just a handful spoke up in support of the their leader's line.
The letter comes ahead of the weekend where Mr Cameron, who argued the UK "will never be safe" unless ISIL is defeated, has encouraged MPs to consider his proposal for the UK to join the US and France in bombing missions to wipe out the extremists.
Labour MPs sympathetic to airstrikes could now feel pressure from grassroots Corbyn supporters to support their leader. One Shadow Minister said MPs were "not going to be bounced" by the letter.
The Huffington Post UK this morning revealed how the Labour leader had been briefed by the UK's top national security advisers on risks to the UK ahead of Mr Cameron making the case to the House of Commons.
The full letter reads:
“The Prime Minister made a Statement to the House today making the case for a UK bombing campaign against ISIS in Syria. A copy of my response has already been circulated.
“We have all been horrified by the despicable attacks in Paris and are determined to see the defeat of ISIS.
“Our first priority must be the security of Britain and the safety of the British people. The issue now is whether what the Prime Minister is proposing strengthens, or undermines, our national security.
“I do not believe that the Prime Minister today made a convincing case that extending UK bombing to Syria would meet that crucial test. Nor did it satisfactorily answer the questions raised by us and the Foreign Affairs Select Committee.
“In particular, the Prime Minister did not set out a coherent strategy, coordinated through the United Nations, for the defeat of ISIS. Nor has he been able to explain what credible and acceptable ground forces could retake and hold territory freed from ISIS control by an intensified air campaign.
“In my view, the Prime Minister has been unable to explain the contribution of additional UK bombing to a comprehensive negotiated political settlement of the Syrian civil war, or its likely impact on the threat of terrorist attacks in the UK.
“For these and other reasons, I do not believe the Prime Minister’s current proposal for air strikes in Syria will protect our security and therefore cannot support it.
“The Shadow Cabinet met today for an initial discussion and debated the issues extensively. We will meet again on Monday, when we will attempt to reach a common view.
“I will get in touch again when we know the timing of the debate and vote.”
The Prime Minister will now give MPs time to consider the contents of the detailed dossier, and will only call on MPs to vote on whether to go-ahead with airstrikes when he can get a “clear majority”.
Much hinges on whether Labour MPs will support the action given the Government’s slim majority and Mr Corbyn’s reluctance to back military action.
The letter was released after the Shadow Cabinet met following the PM's Commons statement to discuss the party's position.
One Shadow Cabinet minister told HuffPost UK: "People are going to stand firm, definitely.
"Colleagues who have made their minds up are not going to be bounced into supporting Jeremy by a load of emails from (left-wing grassroots group) Momentum members over the weekend."
Asked about the fact that Mr Corbyn had not discussed his letter at the Shadow Cabinet meeting, one shadow frontbencher said: "It's amazing. But nothing surprises me any more.
"The Shadow Cabinet was left with the impression that this was just an adjourned discussed and would resume on Monday."
One Shadow Minister said several colleagues had made clear their backing for military action in the meeting, but it was opened by Mr Corybn reading out a prepared text listing his reservations.
When the meeting ended, Mr Corbyn didn't sum up or react to the points made but simply said 'see you next week'.
"It was a normal, grown up discussion, but started very oddly with him reading out his position and ended very abruptly," said one.
Mr Corbyn was - for the first time at a Shadow Cabinet meeting - accompanied at the meeting by all of his inner circle of advisers, including communications chief Seumas Milne, adviser Andrew Fisher, chief of staff Simon Fletcher and senior aide Neale Coleman.
Just a handful of shadow ministers spoke up to support Mr Corbyn's opposition to military action, including Jon Trickett, Diane Abbott and PLP chair John Cryer.
Shadow Chancellor John McDonnell did not speak at the meeting, but earlier today he signalled that his mind had not been changed on the problems of Western intervention.
HuffPost understands that among those to argue for RAF joining the bombing of ISIL in Syria were deputy leader Tom Watson - who stressed he had voted against the Libya air strikes - and Mr Benn, Vernon Coaker, Michael Dugher, Lucy Powell and Angela Eagle.