LABOUR'S deep divisions over airstrikes on Islamic State have increased David Cameron's chances of getting enough Westminster support to extend military strikes from Iraq to Syria.

Shadow Cabinet members expressed fury after Jeremy Corbyn “broke collective agreement and responsibility” by writing to Labour MPs, saying he did not believe the Prime Minister had made a “convincing case” to meet the crucial test that bombing IS in Syria would strengthen national security.

This was seen as a blatant attempt to get local grassroots to pressure Labour MPs to oppose the call for more airstrikes ahead of a Commons vote, possibly next week.

The left-wing Corbynite faction Momentum was said to be preparing to “bombard” Labour MPs over the weekend, urging them to oppose more airstrikes.

Mr Corbyn was in a minority when he spoke against them in the Shadow Cabinet following the PM’s statement to MPs; the “mood” was in favour of supporting Mr Cameron, who earlier told the Commons it was in Britain’s national interest to try to degrade IS in Syria using RAF airstrikes.

Sources said the Shadow Cabinet, which failed to set a collective line, had agreed not to make any detailed comment and reconvene on Monday.

So the party leader’s letter sparked anger. It was branded “provocative” by one Shadow Cabinet member, another described it as "an incredibly hostile act” while a third asked: “How do you deal with someone like this who goes behind our backs?”

Later, one senior figure quipped: “This is war and I’m not talking about Syria.” Talk of resignations was circulating at Westminster.

Amid the Labour division’s Philip Hammond, the Foreign Secretary, said he was encouraged by the initial response to Mr Cameron's statement, saying: "It feels to me that we are building a consensus now for military action in Syria."

With a number of Conservative MPs still opposed to extending airstrikes - and the SNP also against - the PM is likely to need the support of a significant number of Labour MPs if he is to be sure of winning. A free vote on the Labour side now seems inevitable.

Several senior party figures are believed to be moving towards supporting further military action after hearing that defence, intelligence and security chiefs have all warned that Britain is now at greater risk if it did not extend airstrikes to degrade the terrorists’ base than if it did.

Among them is Hilary Benn, the Shadow Foreign Secretary, who said there were "compelling arguments" for increasing military action against IS, also known as Daesh, while Vernon Coaker, the Shadow Northern Ireland Secretary, said: “A significant part of the Shadow Cabinet and the Labour Party are clear; we will need to take airstrikes."

Earlier in the Commons, the PM denied claims that extending them to Syria would make the UK a bigger target for terror attacks, stressing how the country was already in the “top tier” of IS targets.

Pointing out military and intelligence chiefs believed inaction was now a greater threat to national security than action, Mr Cameron stressed how allies, including France and America, wanted Britain to take part in airstrikes, particularly as its precision missiles could play a key role in degrading the terrorists’ base at Raqqa.

Referring to the recent deadly attacks on Paris, the told the Commons: "If we won't act now, when our friend and ally France has been struck in this way, then our friends and allies can be forgiven for asking: If not now, when?"

One key concern among MPs was the issue of ground troops.

Mr Cameron’s claim, based, No 10 said on “robust analysis”, that there were 70,000 moderate forces in Syria was met with a deal of scepticism given there is no unified command of the Free Syrian Army.

Mr Corbyn warned of “unintended consequences” of getting further involved with airstrikes on Syria and said the key question now was “whether extending the UK bombing from Iraq to Syria is likely to reduce or increase that threat and whether it will counter or spread the terror campaign IS is waging in the Middle East”.

Angus Robertson for the SNP, who, like fellow opposition leaders had been confidentially briefed by security chiefs, made clear the Nationalists would not support a vote for military action unless key questions were "satisfactorily" answered.

He noted: “Two years ago the Prime Minister urged us to bomb the opponents of Daesh in Syria. That would probably have strengthened this terrorist organisation. Today, he wants us to launch a bombing campaign without effective ground support in place or a fully-costed reconstruction and stability plan.”

As Mr Cameron and his colleagues over the weekend calculate the level of support for airstrikes, Westminster sources pointed to a motion being drawn up early next week with a vote, possibly, on Wednesday.