Deep divisions in the Labour Party continue to spiral amid warnings of recriminations for MPs supporting David Cameron's plans to extend military action against Islamic State (IS) into Syria.
RAF war planes are gearing up to launch air strikes against the militants in their heartland and parliamentary schedules are being cleared on Wednesday to allow MPs a full debate on the plans.
Shadow foreign secretary Hilary Benn, who is at odds with leader Jeremy Corbyn over intervention, said the danger IS - also known as Isis, Isil and Daesh - posed to the UK meant he was backing the government.
He told BBC Radio 4's Today programme: "I have reached the conclusion that we need to take this action because there is a clear and present threat from Isil/Daesh."
Mr Corbyn was forced to give Labour MPs a free vote in the face of a threatened revolt during a stormy meeting of the shadow cabinet.
The climbdown was seen at Westminster as a crushing humiliation for the Labour leader - who was reportedly shouted at by shadow ministers - after he made clear at the weekend he would decide whether the party's MPs would be whipped or allowed to vote with consciences.
Mr Benn said it was "to the great credit of Jeremy as a leader" that he had allowed a free vote. "People of principle can reach different decisions about how to deal with the threat," he said.
Asked about speculation he could replace Mr Corbyn as party leader, he replied: "I have no interest in leading the Labour Party. I'm doing my job as shadow foreign secretary to the best of my ability."
Shadow energy minister Clive Lewis, a close ally of the leader, warned that if the war "extends with no ends" the party would be looking at who voted in favour of action and "when the blame is apportioned, step forward".
He said: "If there are members of the PLP that want to bomb in Syria and vote with the Tories, then on their heads be it."
Conservative MP David Davis said former Stop the War Coalition chairman Mr Corbyn's decision to allow a free vote had handed a majority to the Prime Minister.
He told Today: "The thing that's triggered this has been Jeremy Corbyn's decision to have a free vote on this, the Labour Party disunity.
"However you want to put it, that's what's given David Cameron a guaranteed majority."
It is thought the first bombing missions could take place before the end of the week if the Commons votes on Wednesday in favour of extending the current action against IS in Iraq into neighbouring Syria.