Jeremy Corbyn is set for a showdown with his own shadow cabinet over Syria air strikes, as a union boss accused Labour MPs of a "sickening" bid to eject the leader.
Mr Corbyn is finally due to announce whether he will order the parliamentary party to oppose the extension of British military action, in a vote that could take place within days.
He again refused to bow to demands for a free vote yesterday, instead merely delivering an impassioned critique of David Cameron's case for attacking Islamic State (IS) in its heartlands.
Arguing that MPs must listen to the "voice" of the party membership - which overwhelmingly elected him - Mr Corbyn said he alone would take the final decision on how to whip the vote.
The defiant appearance on the BBC's Andrew Marr show will have done nothing to ease tensions with his shadow cabinet, amid warnings of resignations if the leader refuses to compromise.
Mr Corbyn is expected to publish the results from a survey of Labour activists' opinions on Syria this morning, before gathering his senior team for a last-ditch effort to hammer out a collective position.
The Parliamentary Party - a large proportion of which is believed to be considering supporting air strikes - will then hold what could be a highly fractious meeting this evening.
Unite boss Len McCluskey has waded into the row, accusing Mr Corbyn's opponents of using Syria as the "thin edge to stage a coup".
In an article for Huffington Post, Mr McCluskey - who endorsed Mr Corbyn's bid for the leadership but has since criticised his performance - made clear that he thought David Cameron's plan to extend air strikes from Iraq to Syria was "illegal and irrational".
"Backbench MPs are even calling on him (Mr Corbyn) to quit for having the temerity to maintain his values and principles, with one even comparing him disgracefully to a 'fuhrer'. That is not open debate, it is abuse and should have no place in the party," Mr McCluskey wrote.
"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 went on: "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."
In his interview, Mr Corbyn dismissed intelligence advice that IS was using its territory in Syria to prepare terror atrocities against Britain, arguing that "those attacks could be planned anywhere".
He also "seriously questioned" the Prime Minister's claim that there are 70,000 moderate Syrian troops to tackle IS forces on the ground, and voiced doubts about their "loyalties".
Mr Corbyn said a unanimously-passed UN Security Council resolution calling for "all necessary measures" against the terrorist groups did not provide justification for military action.
Asked whether Labour MPs - dozens of whom are thought to be considering supporting action - would be given a free vote, Mr Corbyn said: "No decision has been made on that yet, I am going to find out what MPs think."
But he made clear he did not think it was a collective decision for the shadow cabinet:
"It is the leader who decides. I will make up my mind in due course."
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 airstrikes.
He rejected claims that bombing Raqqa and other IS-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 IS in Iraq, and warned that the UK's reputation would be damaged and the population less safe if action was not taken in Syria.
Shadow chancellor John McDonnell, Mr Corbyn's closest ally, and deputy leader Tom Watson are among those who have urged a free vote.
Shadow justice secretary Lord Falconer confirmed there were "significant differences" within the shadow cabinet, and he did not think it would be possible to reach a collective view. A majority of the team is thought to favour air strikes.
"My own view is I don't think this very very important issue should be a situation that forces resignations on people," the peer told the BBC's Sunday Politics.
Pressed on whether he personally was ready to resign, Lord Falconer said: "I don't want to comment on that."
Even MPs who share Mr Corbyn's opposition to air strikes and believe he should whip the vote have expressed despair at his handling of the situation.
Jess Phillips told BBC Radio 4's The World This Weekend: "If the Labour Party as a collective group of people cannot have a consensus position on this, that is very, very lamentable.
"I think it's been handled appallingly."