Divisions within Labour appeared to grow over the question of airstrikes on Syria, as leader Jeremy Corbyn polled party members on the issue and shadow chancellor John McDonnell backed a free vote for Labour MPs.
Mr Corbyn is at odds with many members of his shadow cabinet over the question of whether British fighter jets should bomb targets in Syria, a move he opposes.
But a free vote for Labour MPs looks more likely now, backed as it is by John McDonnell, the shadow chancellor and an ally of Mr Corbyn's.
"There are some issues like going to war that should be above party politics," Mr McDonnell said on BBC Radio 4's Any Questions.
Mr Corbyn appears in for a difficult weekend battling party rebels.
There have already been calls for Mr Corbyn to resign over the issue, while The Times reported that senior Labour MPs have sought legal advice on how to unseat him.
Mr Corbyn further angered rebels by sending out a survey to party members asking for their views on possible airstrikes.
Party members fear he is trying to use his substantial grassroots powerbase to "bounce" the shadow cabinet into backing his stance.
In his letter, 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?"
Former minister John Spellar said the poll was Mr Corbyn exerting pressure on his colleagues.
"He is almost certainly trying to bounce the shadow cabinet into [opposing airstrikes]," he said.
"I think it will have the opposite effect. I think people will be saying, we will not be pushed around like this."
Protesters for Stop the War - an anti-war campaign group which until recently was chaired by Mr Corbyn - turned out in central London on Saturday in opposition to the proposed airstrikes.
David Cameron could bring the airstrike question to the Commons as early as Tuesday, but the prime minister has made clear he will not do so unless he is certain of securing a majority in favour.
Over the weekend Downing Street is expected to brief Labour MPs to shore up their support.
The Telegraph reported that as many as 115 Labour MPs are preparing to back the government's motion.