The prime minister denied claims attacking ISIS would make the UK a bigger target for terror attacks, as he made the case for military action, in the Commons.
He told MPs the UK was already a target for IS - and the only way to deal with that was to "take action" now.
The UK could not "outsource our security to allies" and it had to stand by France, he added.
Cameron also says he will hold a Commons vote on Syria air strikes if he thinks he will win it.
Follow rolling updates on the UK's Syria debate the Nine key questions on Syria had been answered as summarised below
Labour leader Jeremy Corbyn sought assurances the UK would not be dragged into a ground war and asked the PM whether UK air strikes would make any military difference.
He said there was "no doubt" the "so-called Islamic State group has imposed a reign of terror on millions in Iraq, Syria and Libya" and that it "poses a threat to our own people".
But he added: "The question must now be whether extending the UK bombing from Iraq to Syria is likely to reduce, or increase, that threat and whether it will counter, or spread, the terror campaign Isis is waging in the Middle East."
Mr Cameron said "we face a fundamental threat to our security" and could not wait for a political solution, and that doing nothing "could make the UK more of a target for Isil attacks".
"That bomb in Paris, that could have been London. If they had their way, it would be London," said Mr Cameron.
"I can't stand here and say we are safe from all these threats. We are not. I can't stand here either and say we will remove the threat through the action that we take.
"But do I stand here with advice behind me that taking action will reduce and degrade that threat over time? Absolutely and I have examined my conscience and that's what it is telling me."