Labour MPs are to be offered a free vote on David Cameron's proposals for air strikes against the Islamic State terror group in Syria.
It is understood that leader Jeremy Corbyn will make it clear that the party's official policy is to oppose the military action, but will not impose a whip directing them to vote against.
The party's press office declined to comment on a Guardian report that Mr Corbyn had agreed the deal during talks with deputy leader Tom Watson and other senior shadow cabinet members over the weekend.
But a senior source confirmed that a free vote would be offered, as the shadow cabinet gathered in Westminster to discuss an issue which has exposed deep rifts at the heart of the party.
A free vote would clear the way for Mr Cameron to call a vote on air strikes as early as Wednesday with confidence that he will obtain the "clear majority" which he has said is needed.
But it is thought that Mr Corbyn may appeal for the Prime Minister to delay a vote while the concerns of Labour MPs are dealt with.
Mr Corbyn has written to Mr Cameron urging him to allow a full two days of debate on the air strikes proposal.
In his letter, the Labour leader said: "In the view of the Opposition on a matter of such critical importance there must be full and adequate time for any debate in the House and only a full two-day debate would ensure time for all members who wish to participate to be able to do so.
"As has happened previously a one-day debate would inevitably lead to important contributions being curtailed. It is incumbent on us all to ensure the country feels there has been the fullest parliamentary discussion of what you have rightly described as a highly complex situation."
Senior Conservative backbencher David Davis backed Mr Corbyn's "eminently sensible" call for a two-day debate.
"The decision to go to war is by definition a matter of life and death," said the former shadow home secretary.
"If the Government is seeking the authority of Parliament to undertake such a policy it should do so on the basis of the best-informed, and most carefully considered, debate.
"It is perfectly possible to fit a two-day debate in before Christmas while also giving the relevant select committees the time to review the issue in detail.
"I strongly recommend that the Government accept this proposal and make arrangements accordingly."
A Number 10 spokesman said discussions were under way within government on the wording of a draft motion - covering not only military action but also work on counter-extremism, the diplomatic and political process and the ongoing humanitarian effort - but stressed there was so far no timetable for it to be put before MPs.
Downing Street confirmed MPs from all sides of the House were being offered briefings on the Government's case for UK involvement in air strikes.
Privy counsellors from Labour and other opposition parties were receiving briefings at a higher security level from security and government officials.
Shortly before the potentially-explosive meeting of the shadow cabinet, Labour released analysis of a survey of members apparently indicating overwhelming opposition to extending the RAF's bombing campaign from Iraq into Syria.
The survey, ordered by Mr Corbyn, received 107,875 responses, of which 64,771 were confirmed as full individual Labour Party members.
An initial analysis of 1,900 responses showed 75% opposed to bombing, 13% in favour of the air strikes and 11% undecided.
One of Mr Corbyn's closest allies in the shadow cabinet, Diane Abbott, said "the party as a whole" was opposed to bombing and "looking to Jeremy to show leadership".
But Mr Corbyn is facing the prospect of a rebellion by senior figures, including members of the shadow cabinet, if he tries to impose a party line.
Shadow international development secretary Ms Abbott said: "I think it's quite an evenly balanced argument. Jeremy doesn't want resignations, he doesn't want to lose valued colleagues.
"But the truth is we now know the party as a whole, in the country and even within the PLP (Parliamentary Labour Party), is opposed to these bombings and they are looking to Jeremy to show leadership."
She told the BBC: "Frankly, I think the threat of mass resignations has been exaggerated but we will have to see. But what is clear is that a decision about the whipping lies with the leader."
Shadow chancellor John McDonnell, Mr Corbyn's closest ally, is among those who have urged a free vote but played down expectations of resignations.
"I think we will arrive at a common position and people will hold together," he told Sky News.
Mr McDonnell said the leader's position "not to bomb" was also the stance of "the majority of our party members and quite a few Conservative MPs now".
Pete Willsman, a constituency party representative on Labour's ruling National Executive Committee, said there should be a three-line whip requiring the party's MPs to vote against military action.
"They are not there to vote with the Tories. They are there to vote with Labour and follow the Labour conference policy," Mr Willsman told BBC Radio 4's World at One.
"Who do these people think they are? It's just amazing, they are speaking to the press all the time, they are totally disloyal.
"It's just appalling but Jeremy is so soft he wouldn't say boo to a goose, he's so good-natured. I would be much tougher with these people and the NEC would be much tougher on them."
Labour MP Angela Smith said she was ready to defy a three-line whip to back air strikes, telling World at One: "Effective leadership requires careful consideration of the evidence when it comes to an important decision, and it requires a due regard for the views of the public, the views of people who vote for you and the views of the elected representatives.
"We are in danger of disregarding quite a lot of that, and in that case I would feel free to vote the way my conscience tells me to vote, against a three-line whip."
The Penistone and Stocksbridge MP said Labour MPs were "in danger of being bullied" by opponents of military action who have taken to social media to demand they vote against.
Chancellor George Osborne accused opponents of air strikes of "handing over" responsibility for national security.
"This is not about the internal politics of the Labour Party, this is about the internal security of our country," he said.
"We want to take the fight to this terrorist organisation. We have got the capabilities, we've got our allies like the French and the Americans saying 'join us in this endeavour', and we know as a country we have never ever handed our security over to anyone else.
"We have always been prepared to defend ourselves and we should do so on this occasion."
Lindsey German, of the Stop the War Coalition, said she was sorry to hear Labour MPs would have a free vote, adding the Prime Minister was more likely now to win support for a bombing campaign in Syria.
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Former Respect Party MP George Galloway said: "I am sorry to say Jeremy Corbyn has made a fateful tactical, strategical and moral error of grave magnitude.
"It will, alas, mean that the bombs falling on Syria will be Labour-Tory bombs. Cameron's war made possible via a free vote of Labour MPs."
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Labour MP John Woodcock said Mr Corbyn's position added to the confusion and the poll of members was "completely not credible".
Mr Woodcock, who is in favour of air strikes, said: "Particularly after the last couple of weeks, where I think by anyone's estimation Labour has seemed confused in the face of such a serious issue as intervention, we have got a responsibility to put that right now.
"If the party leader makes a call that actually throws fuel on to the fire, all it will do is distract further from the actual issue at hand which is how does the UK best defeat a group that is systematically killing, raping and enslaving everyone who does not bow down to their very precise, medievalist perversion of Islam."
He questioned how Mr Corbyn could say that the party's official position was against the strikes. He criticised the poll, claiming it excluded many Labour members who were not active on social media or email, and questioned the timing of the release of the results.
"I just imagine what Jeremy Corbyn and Diane Abbott and John McDonnell would have had to say if a past Labour leader had deployed those kind of tactics one hour before a very important meeting of the Cabinet before military intervention."
He questioned whether the vote would be genuinely free, with "certain people floating around the edges and warning of the dire consequences to their political future if they vote in the way that they believe to be right, which just happens to be in disagreement with the leadership".
