The prime minister is taking lots of interventions on MPs - although mainly on Syria, rather than than Iraq. Former Northern Ireland Secretary Peter Hain presses Mr Cameron over earlier remarks that the government would authorise action to prevent a humanitarian crisis before consulting parliament first. Mr Cameron says he supports the convention of seeking parliamentary approval first but stresses that it is important for the government to reserve the right to act without it, in the event of a critical threat to the national interest or to prevent a catastrophe.

Mark D'Arcy
Parliamentary correspondent

Labour MP Peter Hain pounces on the PM's comment that he would act without parliamentary approval to address a humanitarian catastrophe. A significant moment. The noise level in the Chamber seems to be edging upwards.

Robert Halfon tweets, external: PM says international Law under UN Responsiibility to Protect also justifies intervention

Setting out what a successful outcome would look like, Mr Cameron says it would result in a stable Iraq and Syria with IS "degraded and destroyed" as a serious terrorist threat. But it will not happen quickly, he cautions - and repeats assurances that UK ground troops would not be deployed.

The prime minister tells the Commons there is "no question" about the legality of the military action being proposed by the UK government. He says there is a clear basis in international law, with a request from the Iraqi government. It is "morally right" to ask the Armed Forces to take part in international air strikes, "and I believe we should do so now", he adds.

Labour backbencher David Winnick presses the PM to say whether the action he is proposing will be successful in the UK's objectives. He says action could last for years. Mr Cameron tells him IS "have already declared war on us" and that there isn't "a walk on by option".

Mark D'Arcy
Parliamentary correspondent

For the first time a bit of hostile murmuring as Green MP Caroline Lucas intervenes to call for more effort towards a diplomatic solution.

Conservative MP Sarah Wollaston, who voted against action in Syria last year, told BBC News: "This time I will be voting in favour of action. I think the situation now is very different from last year.

"I think we're seeing an unfolding genocide and I think we have a duty not to walk the other side of the road. I also think that ISIS is a very clear threat to us here in the UK irrespective of whether we get involved."

David Cameron is flanked by his foreign secretary, Philip Hammond, and the Lib Dem Deputy PM Nick Clegg. He tells MPs they must weight up the consequences of action and inaction. "If we allow Isil to grow and thrive there's no doubt in my mind that the level of threat to the country would increase."

Image source, Getty Images

Mr Cameron tells the Commons that President Barack Obama has "made clear" he wants Britain to join the air strikes. The PM says UK involvement is also protecting British national security, "so it's right for us to act".

On why military action is necessary, Mr Cameron tells the Commons there is "no realistic prospect" of defeating IS without it. The Iraqi government wants more direct assistance, he goes on to say. "They need our military help and it is in our interest and theirs to give it." Bob Ainsworth intervenes to say the Iraqi Army must have the will to defend the Sunni Muslim areas of the country. Replying, Mr Cameron says there is "no doubt" in his mind that the government understands this point - but the UK must make clear that any help is conditional on the Iraqi government defending "all of your people".

Mr Cameron says the situation in Syria is "more complicated" because of its "brutal dictator" President Assad and the civil war that has been ongoing for the past three years.

Former Tory foreign minister Alistair Burt asks the PM whether the UK would consider giving military hardware to the Free Syrian Army to take on President Assad and IS. They need our help, he urges. Mr Cameron says the government has supported them with advice, training and non-lethal equipment "and I am not proposing a change to that today". He says there is a "strong case" to do more in Syria - but tells MPs he did not want to bring a motion before the Commons which had "no consensus".

Mark D'Arcy
Parliamentary correspondent

The PM is not ploughing on regardless - he's taking interventions from sceptics and supporters. MPs Dennis Skinner, Hazel Blears and Edward Leigh have all stood up.

David Cameron says action abroad must include humanitarian efforts, diplomatic efforts to engage the widest possible coalition of countries against IS, and political efforts to support an "inclusive" and "democratic" governments in Iraq and Syria. But I do believe that our military has an indispensible role to play, he adds.

Another intervention, this time from senior Conservative Sir Edward Leigh, who asks whether air strikes alone can "roll back" IS, "or is this just gesture politics?"

Mr Cameron says the cause of the problem is the "poisonous narrative of Islamist extremism" and adds that Muslims must "reclaim their religion from these extremists".

Veteran Labour MP Dennis Skinner intervenes to ask "how long will this war last and when will mission creep start". Mr Cameron tells him it will take "years" but "we must be prepared for that". He stresses that the coalition against IS includes Arab states and the UK must be prepared to play its part.

The prime minister says the government has a "clear plan" to combat the Islamist group. He takes MPs through actions the government is taking to defeat all forms of Islamic extremism at home and abroad.

Mr Cameron says IS is a threat to national security. It is a terrorist organisation unlike any other we have known before, he says, and condemns the "brutality" of the group's actions. It is also backed by billions of dollars and has captures an arsenal of the most modern weapons, he tells MPs. "This is not a threat on the far side of the world."