Jeremy Corbyn is under pressure to allow Labour MPs a free vote on launching military action against Isis in Syria.
Britain is currently carrying out airstrikes against Islamic militants in Iraq. David Cameron has indicated he will ask parliament to allow him to extend those strikes over the border into Syria.
Corbyn has voiced deep skepticism about the idea of an increased bombing campaign, but many Labour MPs agree with the prime minister that strikes against Isis need to be expanded.
One Labour MP told The Huffington Post UK that "he's got to allow a free vote: If he doesn't he could lose half his shadow cabinet".
A shadow cabinet minister told the Sunday Times: "There is a majority in the Commons for airstrikes in Syria if Cameron has a proper plan for targeting Isis. You would get half the shadow cabinet supporting it."
A free vote would mean Labour MPs would be allowed to vote with the government even if Corbyn argued against Cameron's position.
Backing intervention would put Labour MPs at odds with their new leader, who has just ended a stint as chairman of the Stop the War Coalition.
Corbyn was due to speak at the organisation's annual general meeting in central London but pulled out to prepare for Labour's party conference.
Refugees Pepper Sprayed In Serbia
Shadow foreign secretary Hilary Benn today insisted Labour will not scrap Trident or leave Nato. Corbyn opposes the replacement of the nuclear weapons system and has questioned the role of the international military alliance.
But Benn said Nato was the "conerstone of our security" and insisted he did not see Britain withdrawing. He told BBC One's Andrew Marr Show: "My view is that we need to maintain an independent nuclear deterrent."
He added: "We live in a differently dangerous world now. We need a continuous at sea deterrent. We need to do it in the most cost effective way and that is the view which the Labour party, including the Labour party conference has taken for many years."
Asked if he could serve in the shadow cabinet if the party voted to leave Nato and get rid of Trident, he replied: "I don't think that is going to happen."
Pressed over whether he would support military action in Syria, he told the BBC: "We have said, consistently, if the Government has a proposition we need to look at what that is."