The Prime Minister and the leader of the Opposition have set out their respective cases for and against bombing ISIS in Syria.
A 10 hour House of Commons debate is taking place now and will end with a vote on whether the UK joins others such as France, the US and Russia in bombing targets in Syria.
The government motion would authorise air strikes “exclusively” against ISIS in Syria.
Bombing missions are likely to begin within days if the Commons delivers what Mr Cameron hopes will be a majority after Mr Corbyn abandoned attempts to impose his opposition to air strikes on Labour and allowed his MPs a free vote.
Calling on MPs to “answer the call from our allies” and take action, Mr Cameron said: “The House should be under no illusion that these terrorists are plotting to kill us and to radicalise our children right now.”
He said MPs faced a simple question: “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 to attack us?”
The prime minister also defended his controversial claim that there were 70,000 moderate opposition fighters in Syria, saying it was the estimate of the Joint Intelligence Committee – the UK’s senior intelligence body.
He said the majority were members of the Free Syrian Army and that there were a further 20,000 Kurdish fighters with whom Britain could also work.
He told MPs the forces were “not ideal, not as many as we would like, but they are people we can work with”.
Meanwhile, Labour leader Jeremy Corbyn told MPs that the prime minister understands public opposition to his ill thought-out rush to war is growing – and wants to hold the vote before it slips from his hands.”Whether it’s the lack of a strategy worth the name, the absence of credible ground troops, the missing diplomatic plan for a Syrian settlement, the failure to address the impact on the terrorist threat or the refugee crisis and civilian casualties,” he said.
“It’s become increasingly clear that the prime minister’s proposals for military action simply do not stack up.”
Corbyn also disputed Mr Cameron’s claim about ground troops, saying it was “quite clear there are no such forces” and only extremists would take advantage of the strikes against IS.
Mr Corbyn’s aides say as many as 90 Labour MPs could back the government – and with both the Democratic Unionist Party and the Liberal Democrats backing action Mr Cameron is expected to win parliamentary approval for the UK to intervene militarily in the four-year conflict in Syria.
However, at least 110 MPs from six different parties – including the SNP, which opposes action – have already signed up to an amendment seeking to block air strikes.