Jeremy Corbyn should order his MPs to vote against air strikes against Islamic State (IS) in Syria, according to a a key shadow cabinet ally.
Labour 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 on Sunday, instead merely delivering an impassioned critique of David Cameron's case for attacking 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.
Shadow international development secretary Diane Abbott, a close colleague of Mr Corbyn on the Labour left, said the final decision was for the leader but insisted "public opinion is moving towards us in opposing the rush to war".
She added: "It's a matter for the leader what the whipping will be, but we are a party of government and a party of government has to have a position on matters of peace and war.
"The problem about a free vote is it hands victory to Cameron of these air strikes, it hands victory to him on a plate. I don't think that's what party members want to see."
Ms Abbott told BBC Radio 4's Today programme: "Party members and, increasingly, the country, want to see us oppose these air strikes - which are not the solution - with every sinew of our being, and that would mean a three-line whip."
Dozens of Labour MPs are thought to support air strikes, including a majority of the shadow cabinet, and imposing a whip could trigger a wave of resignations.
Ms Abbott said: "Jeremy does not want to have either sackings or resignations, none of us want to lose valued colleagues.
"However, I think the party and increasingly the public would be disappointed if we didn't oppose these air strikes to the limits of our ability."
Mr Corbyn used an appearance on the BBC's Andrew Marr Show to insist the decision on how to approach a vote would rest with him.
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 on Monday 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 on Monday 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.
"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 "those attacks could be planned anywhere".
He also "seriously questioned" the Prime Minister's claim 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 MPs 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."