David Cameron is on his feet addressing MPs, setting out the terms for any UK military involvement in air strikes against IS militants. He says the question is about how to address "the threat posed" by IS.

We're getting the first pictures from the House of Commons. The green benches are packed.

Mark D'Arcy
Parliamentary correspondent

Today's timetable is helpfully laid out here: a Commons vote is expected about 17:00 BST (16:00 GMT).

The debate in the Commons will get underway in just a few moments. There is a five-minute time limit on backbench speeches, due to the number of MPs wishing to take part in the debate.

MP Andrew Stephenson tweets, external: Met the Attorney General this morning discussing the legal basis for military action, now heading to the @HouseofCommons for prayers

The BBC's Justin Parkinson has taken a look back at previous Commons debates and votes on war.

The proposed motion to be debated by Parliament comes after a third night of US-led air strikes which targeted IS-controlled oil refineries in Syria. Jets from Saudi Arabia and the United Arab Emirates have joined US forces in the attacks, and the US says more than 40 countries have offered to join the anti-IS coalition.

The BBC's James Robbins has been looking at the role the British military could play if UK action is given parliamentary approval.

House of Lords tweets, external: Lords debates developments in #Iraq from 11.30 http://goo.gl/GaRs7Q, external

The government does not have to seek the approval of MPs to begin military action - but it has become customary to do so since the Iraq war in 2003. Last year, Parliament was recalled to discuss possible military action against Syria over President Bashar al-Assad's alleged use of chemical weapons. It was the 27th recall during a recess since 1948.

Speaking to BBC News, Labour MP Graham Allen - who is opposing UK military action - warns that the UK is going to play into the hands of IS by joining in air strikes "and we will live to regret it, unfortunately". These things can only be sorted out by the Arab nations in the region, he adds.

Prime Minister David Cameron will open the Commons debate at 10:30 BST (09:30 GMT) - it is expected to last seven hours. The House of Lords is also sitting today - from 11.30 BST - to discuss possible UK military action.

Labour leader Ed Miliband said this morning: "I'm not going to make predictions but we will be supporting action against Isil in Iraq today in the House of Commons because Isil is a murderous organisation which threatens the stability of Iraq, the region and a threat to our national interest."

Speaking on BBC Radio 4's Today programme earlier, Foreign Secretary Philip Hammond said: "The plan is to provide air support to the Iraqi government, alongside our allies - the US, France and an increasing number of Arab countries who are going to take part."

IS - also known as Isil or Isis - has seized large parts of Iraq and Syria in recent months. The group has used tactics that have included beheadings of soldiers, Western journalists and aid workers.

The Conservatives, Liberal Democrats and Labour all back action against Islamic State in Iraq, which the coalition says is legal because it was requested by the Iraqi government. The UK government is not proposing any involvement in air strikes on Syria.

The motion being debated, external states that IS is a direct threat to the UK, that the Iraqi government has requested assistance and any action does not extend to ground troops or air strikes in Syria.

Follow our live coverage of the recall of Parliament, as MPs debate whether the UK should join air strikes in Iraq.