David Cameron is expected to make a statement this evening on the next steps on airstrikes in Syria, after Labour MPs were told they can vote with their consciences on military action.

Labour leader Jeremy Corbyn's decision to allow his MPs a free vote appears to clear the way for the Prime Minister to seek the approval of the House of Commons for extending RAF bombing missions against the Islamic State terror group from Iraq into Syria.

A two-hour meeting of the shadow cabinet heard that as many as 43% of Labour MPs - almost 100 of the 231-strong parliamentary party - back airstrikes, against 57% - around 132 - who would follow Mr Corbyn into the No lobby.

If correct, this would provide Mr Cameron with the "clear majority" he wants to avoid a repeat of his defeat in 2013, when he sought approval to launch action against the regime of Syria's president Bashar Assad.

A senior Labour source said both Mr Corbyn and shadow foreign secretary Hilary Benn will speak from the despatch box in the Commons debate, even though they would vote in opposite directions, but denied the party's position was "shambolic".

Labour is pushing for a two-day debate which the party said should not be held until next week at the earliest, and is calling on Mr Cameron to also allow Conservative MPs a free vote.

Labour sources said the party's official policy remained in line with a motion passed by its annual conference in September, which said airstrikes should take place only if four conditions are met.

Mr Corbyn was clear in shadow cabinet that he does not believe these conditions - authorisation from the United Nations, a comprehensive plan for humanitarian assistance and assurances that bombing will be directed solely at IS and will be subordinate to diplomatic efforts to end the war - have been met.

And he said he believes opinion within the party, including the shadow cabinet, is moving against air strikes.

Shortly before the meeting, Labour released analysis of responses to a survey of party members, which appeared to show overwhelming opposition to the extension of military action.

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.

A Labour source said Mr Corbyn would lead the opposition in Parliament to the Government's case for war, while Mr Benn would put the opposite view.

While admitting the arrangement was "certainly unusual", the source insisted: "I don't think it's shambolic. It's taking account of the fact that there are very significant differences of opinion, as we know, in the shadow cabinet about this issue."

Asked if Mr Corbyn was disappointed that he had not been able to persuade his party to oppose the bombing campaign, the source said: "People on both sides of the argument wanted to achieve and hoped to achieve a common view.

"Given that that hasn't been possible across the shadow cabinet - although opinion has clearly shifted in the direction of opposition to bombing, he decided - also on the basis of the consultation of party members - that the right thing to do was go for a free vote and press very hard, make the case, against the Government's rush to war."

A spokesman for Mr Corbyn said: "Today's shadow cabinet agreed to back Jeremy Corbyn's recommendation of a free vote on the Government's proposal to authorise UK bombing in Syria.

"The shadow cabinet decided to support the call for David Cameron to step back from the rush to war and hold a full-two day debate in the House of Commons on such a crucial national decision."

Downing Street confirmed that work has started on drafting a motion for MPs to debate but insisted that there is so far no timetable for a vote, which Westminster insiders believe could come as early as Wednesday.

In a letter to Mr Cameron, Mr Corbyn said: "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 and said it would be "perfectly possible" to make time for it to take place before Christmas.

Labour MP John Woodcock, who is in favour of airstrikes, 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".

Pete Willsman, a constituency party representative on Labour's ruling National Executive Committee, said Mr Corbyn should impose a three-line whip and be "tougher" on MPs who back 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, ahead of the shadow cabinet meeting.

"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."

Number 10 confirmed it had received Mr Corbyn's letter, but said it had not yet responded.

Mr Cameron has been in Paris on Monday and is due to make a further trip abroad on Thursday, limiting the time available for him to take part in a debate and vote this week.

Asked if he was pleased that Labour MPs appeared to have been given a free vote, a Downing Street spokesman said: "Ultimately, it is up to all parliamentarians to come to a conclusion.

"The Government position on this is very clear, the Prime Minister has set out a very clear strategy."

The draft motion being discussed within government covered not only military action, but also the other prongs of Mr Cameron's strategy, involving work on counter-terrorism, the diplomatic and political process and ongoing humanitarian aid, said the spokesman.

A Labour source insisted Mr Benn's position was secure and Mr Corbyn retained "full confidence" in him.

"He is speaking because he is the shadow foreign secretary and this is a free vote and the shadow cabinet is embracing that and giving the chance for the alternative position to be put," said the source.

But "Jeremy will be speaking for majority Labour Party opinion, he will be speaking on the basis of Labour Party policy and he will be speaking as the elected leader of the Labour Party elected with a landslide no previous Labour leader could possibly enjoy".

Calling for the vote to be delayed until next week at the earliest to allow for a two-day debate, the source said there was "no pressing need for the decision to be taken this week except the Government's convenience".