Jeremy Corbyn has been accused of using his grass roots powerbase to try to "bounce" the shadow cabinet into opposing air strikes against Islamic State (IS) in Syria.
The leader has sent out a survey to members asking for their views on the issue, and urging them to respond by the start of next week.
The move could further inflame increasingly bitter tensions with moderates who complain he is trying to push them into bowing to his view.
David Cameron could bring the question to the Commons as early as Tuesday - but has made clear he will not do so unless he is certain of securing a majority in favour.
The chances of Labour MPs being granted a free vote seemed to increase when Mr Corbyn's closest ally, shadow chancellor John McDonnell, insisted politicians had to follow their consciences.
But the veteran left-winger has so far appeared unwilling to accept the idea - and some frontbenchers worry about the electoral consequences of failing to agree a position on a key national security matter.
In his letter to activists, Mr Corbyn wrote: "I do not believe that the Prime Minister made a convincing case that British air strikes on Syria would strengthen our national security or reduce the threat from ISIS.
"When I was elected I said I wanted Labour to become a more inclusive and democratic party.
"So I am writing to consult you on what you think Britain should do. Should Parliament vote to authorise the bombing of Syria?
"Let me know your views, if you are able to, by the start of next week."
As the row continued:
:: French president Francois Hollande called on MPs to back military intervention in Syria in the wake of the Paris attacks that left 130 dead. Thanking the British people for their support, Mr Hollande told a press conference at the Commonwealth Heads of Government Meeting (Chogm) in Malta: "I do hope that the House of Commons will be able to meet the request of Prime Minister Cameron."
:: Three Labour MPs openly suggested for the first time that the situation was so bad Mr Corbyn might have to resign.
:: The Times reported that rebels within Labour have sought legal advice on whether Mr Corbyn could be prevented from getting on the ballot if they forced a new leadership contest.
Appearing on BBC Radio 4's Any Questions, Mr McDonnell made clear his own strong opposition to any Western military action in the Middle East.
"Our involvement in a bombing campaign, it plays into the narrative of Isil, of a crusader invasion," he said.
"It will make the situation worse, not just for us but for the others as well.
"I believe that actually the solution in Syria, and in Iraq as well, is in the hands of the regional powers.
"The bombing will ensure those regional powers will not step up to the plate."
Mr McDonnell denied that Labour was "divided", saying many were struggling to get used to the "new politics" where people could "honestly express their views".
Endorsing the idea of a free vote, Mr McDonnell said the "horrendous mistake" of the Iraq war had been partly due to MPs being "whipped and threatened and pushed" into supporting "something many of them did not believe in".
He said: "I don't believe that is acceptable. There are some issues like going to war that should be above party politics, and I think we are moving to a situation where hopefully in all parties on issues like this a moral conscience should be above the whip as well."
He went on: "On certain issues, the ones really above party politics, we have got to have mature politics in our democracy now. This is a matter of conscience. You are sending people out possibly to die.
"There shouldn't be any party discipline on matters like this. You should follow your own judgement on what you think is best for the constituency and the country."
After a fiery shadow cabinet meeting on Thursday, Mr Corbyn ordered his MPs to go back to their constituencies this weekend and canvas the views of party members.
The Momentum group of Labour activists, set up to help the leader, is expected to lobby politicians intensively. A Stop the War march will take place in London on Saturday - but Mr Corbyn, a vice president of the organisation, is not expected to attend.
In another apparent attempt to draw on Mr Corbyn's strong support among the grass roots, a survey emailed out to hundreds of thousands of members and supporters tonight asked for "views on Syria".
Shadow Cabinet Office minister Jonathan Ashworth posted a website link to the survey on Twitter, saying "consultation always a good thing".
Asked by another user why he had posted a link that allowed non-Labour members to fill it in, Mr Ashworth replied: "Because I think we should take account of views of voters."
Former minister John Spellar said the poll was another way of Mr Corbyn exerting pressure on his colleagues.
"He is almost certainly trying to bounce the shadow cabinet into (opposing airstrikes)," he told Press Association.
"I think it will have the opposite effect. I think people will be saying, we will not be pushed around like this."
Mr Spellar said there was "real anger" about the way the leader was behaving.
"Rushing off to do plebiscites every few minutes is a very odd way to behave. It is no way to run a party," he said.
The MP also suggested the survey would only reach the new Labour members and registered supporters attracted by Mr Corbyn, as the party did not have email addresses for many of the long-term backers.
"You are not going to get a representative picture of the party's views," he added.
Labour backbencher Gavin Shuker told BBC's Newsnight there would be concerns the survey was designed to up the pressure on colleagues.
"Of course we know what Jeremy's position is, so the consultation is not going to change that," he said.
"The concern will be that the consultation is trying to change the views of people who on a matter of principle simply cannot see eye to eye."
Mr Shuker said it was "untenable" for the party not to have a collective position on air strikes, and it might be better for frontbenchers to resign.
"I am someone who would oppose military action in Syria right now, but I think it is frankly untenable that we as a party cannot express a collective view on the issue," he said.
He said that could mean "resignations or abstentions of frontbenchers".
"Jeremy needs to command the support of the people around him, or we are going to be nowhere as a party," Mr Shuker added.