Britain's Cameron says time to bomb militants in Syria

"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, a copy of which was seen by Reuters.

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".