David Cameron has repeatedly said he would not extend the current RAF operations against IS in Iraq – where it is operating at the invitation of the Baghdad government – unless he received the backing of Parliament.
In practice - with up to 30 Tory MPs likely to rebel against such a motion- that means he will almost certainly need the support of Labour to get it through the Commons.
Jeremy Corbyn has said he could not envisage 'any circumstances' in which he would send British troops abroad
However, at a leadership hustings on Thursday, Mr Corbyn – who has repeatedly voiced his opposition to air strikes – underlined his opposition to military interventions saying he could not envisage "any circumstances" in which he would support the deployment of British troops abroad.
Admiral Lord West of Spithead, a security minister in Gordon Brown's government and a former head of the Royal Navy told The Daily Telegraph that if Mr Corbyn won "it is unlikely that David Cameron will go to Parliament again because he will have people on his side voting against him. He doesn't want to risk another defeat".
General Lord Dannatt, a former head of the Army, told the paper a Corbyn victory "will make it much harder and far less likely for David Cameron to be able to get a vote for air intervention over Syria".
Adam Ingram, a former Labour armed forces minister, told the Telegraph that Mr Corbyn's opposition to bombing in Syria would "split not only the Labour Party but a large part of the Labour-supporting public, because the vast majority of British people understand that we have to tackle evil when we see it".