Prime Minister David Cameron said on Thursday it was time for the UK to join air strikes against Islamic State in Syria because Britain cannot "subcontract its security to other countries".
Many Britons are wary of entering into another war in the Middle East after Western intervention in Iraq, Afghanistan and Libya failed to bring stability to the region and, some believe, even led to the rise of militants groups such as Islamic State.
But, after Islamic State claimed responsibility for killing 130 people in Paris, some members of parliament who were reluctant to launch further military action in the Middle East now feel it is needed to protect Britain from such attacks.
Cameron lost a vote on air strikes against Syrian President Bashar al-Assad's forces in 2013 and must persuade some wary members of his own Conservative Party and in the opposition Labour Party to back him if he is to win parliament's support for military action.
After setting out his case, Cameron appeared to have persuaded at least two of 30 party "rebels" who voted against him in 2013, and his foreign minister, Philip Hammond, later said the government was now "building a consensus now for military action".
"We do not have the luxury of being able to wait until the Syrian conflict is resolved before tackling ISIL (Islamic State)," Cameron wrote in a response to parliament's Foreign Affairs Committee, which had said a policy to extend air strikes was "incoherent" without a strategy to defeat the militants.
"It is wrong for the United Kingdom to subcontract its security to other countries, and to expect the aircrews of other nations to carry the burdens and the risks of striking ISIL in Syria to stop terrorism here in Britain," he added.
But Labour leader Jeremy Corbyn, a veteran anti-war campaigner, wrote to his lawmakers later on Thursday to say Cameron had not made a convincing case. "I do not believe the prime minister's current proposal for air strikes in Syria will protect our security and therefore cannot support it," he said in the letter.
Corbyn said his team of senior lawmakers had debated the issue extensively during a meeting on Thursday and would meet again on Monday to "attempt to reach a common view".
Cameron said in his 24-page response that the campaign against Islamic State was entering a new phase, focusing on command and control, supply lines and financial support - something that suited Britain's capabilities.
Drumming up support
Fearful of losing standing on the world stage, Cameron said Britain should respond to requests from allies, including the United States, but said he would not put a vote to parliament unless there was a majority backing action.
He said he did not want to hand Islamic State a "propaganda coup" by losing a vote. The government has not set a timetable for any vote but Cameron said earlier this week that parliament would be able to consider his case over the weekend, prompting many to expect he could push for a vote as early as next week.
Cameron told some lawmakers, who fear joining the air strikes over Syria would make Britain more of a target, that with the threat to the country already as high as it could be, the only way of reducing it would be to "degrade" Islamic State.
British politicians are keenly aware of public opinion over whether to launch air strikes on Syria. A poll by YouGov this week said 59 per cent of people would approve of such strikes, compared with 58 per cent a week earlier.
After Cameron's statement, the Conservative chair of the Foreign Affairs Committee, Crispin Blunt, said he now believed parliament should support the prime minister's judgement "that the United Kingdom should play a full role in the coalition".
Another Conservative lawmaker, Sarah Wollaston, said she had changed her mind and was now in favour of air strikes. But others, in Labour and the Scottish National Party, were less convinced, saying the prime minister had yet to present a clear peace plan for Syria after the military campaign. Others feared that air strikes would pave the way for sending in ground troops, which Cameron denied.
Cameron is hoping to find some support among Labour lawmakers, who are deeply split over Corbyn's anti-war stance. Breaking with a British political tradition of using a "party whip" to maintain parliamentary discipline, Corbyn's finance spokesman said Labour was considering allowing its lawmakers to vote as they wish. "In these sort of issues of conscience it is better to allow MPs to make their own minds up," John McDonnell told BBC television ahead of Cameron's statement.