Mr Grieve says he is "no doubt" that there is a legal basis for action in Iraq - and that there could also be a legal case for action in Syria, to protect the population in northern Syria from IS.

He says this could be possible without an UN Security Council resolution - as the opposition is calling for. While acknowledging that action in Syria would be "more challenging", Mr Grieve says this is not a reason for doing nothing.

The government's former attorney general, Dominic Grieve, speaking from the Conservative benches, begins by sounding a note of caution on military action: that simply bombing IS in Iraq will not eradicate the threat it poses. But he says he finds it difficult "to sit on our hands" when IS continues to perpetrates its "barbarous crimes".

Speaker John Bercow announces that he has cut the time limit on MPs's speeches to four minutes, to try to fit in everyone who wants to speak.

Tom Walker in London: "No, no, no, no, no.... This really has nothing to do with the UK. Without previous involvement, the middle east would be a much more stable place. All this is doing is providing more reason for extremists to keep on threatening our own country."

The Political and Constitutional Reform Committee has produced several reports on Parliament's role in conflict decisions, the most recent of which, Parliament's role in conflict decisions: a way forward, external, was published on 27 March 2014.

It calls for a draft resolution clarifying and formalising Parliament's role. The government has not yet responded to the committee's report.

Crossbencher Baroness Murphy writes for the Lords of the Blog website, external: "I am always uneasy when all political parties agree and I am uncertain about the wisdom of joining what may turn out to be a half-hearted attempt to rid the world of a murderous organisation."

She expresses concern that the outcome will be "unpredictable" and that "there are multiple interlocking problems in the Middle East that this war will not solve".

Bonnie Greer, author

tweets:, external #Labour MP Frank Dobson: "When I began in #HoC,there were men reluctant to send other people's children to war.They'd been there." #Syria

So far in the debate, the consensus appears to be overwhelmingly in favour of the government motion.

Tory Sir Gerald Howarth asserts that it is in the British national interest to join in air strikes against Iraq, and says the case for action in Syria is "quite strong". But he echoes points made by other MPs that military action alone is not the answer to defeating the ideology.

Labour and Dudley North MP Ian Austin distances himself from his party leadership's position on Syria.

Ed Miliband says a UN Security Council resolution should be sought before agreeing to join in air strikes against IS in Syria - which are currently being carried out by the US and allies.

But Mr Austin says Parliament must avoid giving the "impression" that Russian President Vladimir Putin "has a veto" over any such action. He adds that the West's "failure" in Syria has created a "vacuum" which has enabled IS militants to become so strong.

Baroness Browning, the former Conservative MP and one-time Home Office minister, predicts this will be "the beginning of something which is going to last much longer" and that the government will need to return to the House with a separate motion on Syria.

Liberal Democrat Sir Nick Harvey offers his support to the government. He says IS is a "formidable threat" with a "very sophisticated" weaponry. But he says his "greatest misgiving" about the debate is with regards to Syria - and asserts that action against IS in Syria would be a different case entirely.

The Bishop of Derby, the Right Revd Dr Alastair Redfern, advises "we need to be much more proactive" about discussing how to lead "the good life" across different faiths, political perspectives and communities.

As MPs and peers debate the UK's involvement in military action in Iraq against IS, the US has been working to build a coalition to counter the spread of IS. How do other key countries view IS and how are they contributing to the effort against it?

Andre in London: "It is sad we have to resort to a violent response but it seems inevitable with IS. I worry we may end up sending troops in which has already cost the country a lot of good men and women. IS also need to realise the greater their barbaric acts the greater the worlds resolve."

What force does international law allow? Marc Weller, Professor of International Law at the University of Cambridge, explains which questions arise as to the basis of military action according to international law.

He explains that the legal evaluation differs where operations against IS in Iraq on the one hand and in Syria on the other are concerned.

Seeking to set out the challenges that lie ahead, Mr Duncan says this is a conflict without borders and the advance of "non-state actors" from a "poisonous viral movement".

We can resolve to beat them but it is not the same as fighting a country, the Rutland and Melton MP cautions.

He insists David Cameron must be given the flexibility and discretion to respond to events on the ground as the government sees fit. And he stresses the need for military action to go hand-in-hand with diplomatic, social, religious and financial action.

Former International Development Minister Alan Duncan says the path ahead looks neither easy nor conclusive.

He says the UK is justified to deploy its armed forces but adds that the strategic objective is "much more difficult to shape than in the past".

The rise of IS has taken everyone by surprise, and the UK's well of understanding about the region has run rather dry, Mr Duncan tells MPs.

Lord Ramsbotham, a crossbencher and retired British Army officer, says he is "concerned" by the suggestion that "air power is the answer".

He points out when we refer to "boots on the ground" we should think not only of Army boots but those involved in wider diplomatic and educational efforts to counter IS influence in Iraq. He adds there should be a force commander to head up the military coalition.

Another voice in support of the motion. Gisela Stuart - Labour MP for Birmingham Edgbaston - says there is a "myth" developing that IS is "undefeatable" and cannot be contained. She predicts MPs will return to the subject of action Syria - but says it is "incredibly foolish" to think that UK involvement in air strikes is some kind of magic answer there.

Lib Dem Lord Avebury agrees "air strikes are not sufficient by themselves" and raises the possibility of deploying infantry on the ground. He wants to know whether talks are taking place with Russia about providing military support.