Britain "cannot afford" to stand aside from the fight against the so-called Islamic State terror group in Syria, David Cameron has said.
The Prime Minister set out his case for the extension of RAF air strikes from Iraq into Syria in a written response to a parliamentary committee which had urged caution over the move.
Mr Cameron acknowledged that air strikes alone will not be enough to defeat IS, but said they would help to degrade the group's military capability and halt its advance.
He rejected the idea that joining the US, France and other nations in bombing IS in its Syrian strongholds would put Britain at risk of Paris-style terror attacks, saying that the threat to the UK was already "very high".
And he told the House of Commons Foreign Affairs Committee: "One thing is clear: the threats to our interests and to our people are such that we cannot afford to stand aside and not to act."
Mr Cameron then made a statement to the Commons, in which he admitted that IS - also known as Isis, Isil and Daesh - could not be defeated by airstrikes alone. But he argued it was a key part of a wider "comprehensive" strategy to deal with the threat.
Mr Cameron said he would not call a vote in the Commons on airstrikes in Syria until he was sure there was a clear majority in favour of action as defeat would be a "publicity coup" for IS.
He told MPs that Britain must judge whether inaction in Syria carried greater risks than action.
And he added: "The military advice and diplomatic advice and the security advice all says that the risks of inaction are greater."
Addressing concerns that joining airstrikes in Syria would put Britain at risk of Paris-style terror attacks, the PM said that security agencies agreed that the UK was already "in the top tier of countries that Isil is targeting".
"The only way to deal with that reality is to address the threat we face and to do so now," he said.
Mr Cameron acknowledged that airstrikes alone would not be enough to defeat IS, but said they would help moderate Syrian forces which deploy an estimated 70,000 troops on the ground.
The full answer to the threat from IS would not be delivered until there is a new Syrian government which is genuinely representative of all the country's people, he said.
Mr Cameron said his "first responsibility" and that of all MPs was to "keep the British people safe".
"The reason for acting is the very direct threat that Isil poses to our country and our way of life," he said.
"They have already taken the lives of British hostages and inspired the worst terrorist attack against British people since 7/7 on the beaches of Tunisia."
Mr Cameron said seven attacks over the past year had been linked to IS or inspired by its propaganda.
"I am in no doubt that it is in our national interest to stop them. And stopping them means taking action in Syria, because it is Raqqa that is their headquarters," he said.
He added: "We shouldn't be content with outsourcing our security to our allies.
"If we won't act now, when our friend and ally France has been struck in this way, then our friends and allies can be forgiven for asking: If not now, when?"
Mr Cameron said he was pursuing an "Isil-first" strategy while continuing to work for a long-term settlement for Syria.
"We know that peace is a process, not an event and I am clear that it can't be achieved through a military assault on Isil alone, it also requires the removal of Assad and a political transition.
"But I am also clear about the sequencing that needs to take place: this is an Isil-first strategy."
He indicated that any Commons motion on extending the bombing campaign to Syria would explicitly recognise that "military action is not the whole answer".
He told MPs: "There will not be a vote in this House unless there is a clear majority for action, because we will not hand a publicity coup to Isil."
But he added: "We do face a fundamental threat to our security. We can't wait for a political transition, we have to hit these terrorists in their heartlands right now and we must not shirk our responsibility for security or hand it to others.
"Throughout our history the United Kingdom has stood up to defend our values and our way of life. We can, and we must, do so again."
France thanks Ireland for solidarity
Meanwhile, France’s Ambassador to Ireland has thanked the Irish people for their reaction to the attacks in Paris.
Addressing the EU Affairs Committee this afternoon, Jean Pierre Thebault said "the scale and level of organisation of the attacks was exceptional" and "marked the worst attack on France since the Second World War".
He said: "I want to wholeheartedly acknowledge the mass solidarity and support of the Irish people. We were overwhelmed by the huge friendship and huge sorrow shown by the Irish people."
He outlined that every EU country must set up agencies to track down illegal financial flows of terrorist organisations.
He said that he has personally visited the Irishman who was injured in the attacks.