Britain's parliament is likely to vote on December 2 to approve British air strikes against Islamic State militants in Syria.
Prime Minister David Cameron has urged lawmakers from all parties to back his call for attacks on the militants.
As debate opened, Cameron said Britain could make a "real difference" to the U.S.-led campaign to destroy the militants.
"The threat is very real," Cameron told a packed house of parliament at the start of a 10-hour debate.
"The question is this: Do we work with our allies to degrade and destroy this threat and do we go after these terrorists in their heartlands from where they are plotting to kill British people, or do we sit back and wait for them?"
The motion says military action against Islamic State "is only one component of a broader strategy to bring peace and stability to Syria."
Cameron suffered a parliamentary defeat in 2013 over plans to bomb the forces of Syrian President Bashar al-Assad.
Drawing on that experience, Cameron made it clear he would not bring a vote to parliament if he did not think he could win it.
Jeremy Corbyn, a veteran antiwar campaigner elected as the opposition Labour leader in September, opposes the move, but said he would allow his lawmakers to vote according to their conscience -- breaking with a tradition for leaders to instruct lawmakers how to vote on big decisions.
The vote is expected late on December 2 after 10-1/2 hours of debate in parliament.