MPs will vote later on whether the UK should join US-led air strikes on Islamic State (IS) in Iraq.
The Conservatives, Liberal Democrats and Labour all back action, which the coalition says is legal because it was requested by the Iraqi government.
Prime Minister David Cameron will open the debate at 10:30 BST (09:30 GMT) - it is expected to last seven hours.
IS has seized large parts of Iraq and Syria in recent months, killing several Western hostages.
The UK government is not proposing any involvement in air strikes on Syria.
On Thursday, about 250 people protested outside the gates of Downing Street against the possibility of military action in Iraq.
Some MPs also oppose the military action.
Labour's Diane Abbott, Jeremy Corbyn and John McDonnell signed a letter to the Guardian which said bombing Iraq would 'only exacerbate the situation'.
'We urge the government to rule out any further military action in Iraq or Syria,' they said.
The Commons has been recalled from its four-week break for the Scottish independence referendum and party conferences to deal with the developing situation.
The government does not have to seek the approval of MPs to commence military action, but it has become customary to do so since this first happened over the Iraq war in 2003.
by Sean Curran, parliamentary correspondentWhen David Cameron stands up at the dispatch box to open the debate about military action he will know that the result of the Commons vote is not in doubt.
We are not about to see a repeat of the scenes at the end of August last year when a government's proposal to consider military action against Syria was rejected at the end of another emergency debate.
This time there's agreement between the three largest parties at Westminster, but that doesn't mean there won't be a real debate.
Some MPs are opposed to military action and will argue and vote against the motion.
That motion spells out that the Commons is not being asked to endorse air strikes in Syria and we can expect questions and concerns about what would happen in the future.
Its motion specifically rules out any attacks on IS in Syria, following the Commons' rejection of joining in air strikes in that country last year. Any action there would require a separate vote.
Downing Street said UK forces could join the bombing of targets in Iraq after the Iraqi minister of foreign affairs wrote to the United Nations seeking international assistance.
Foreign Secretary Philip Hammond reiterated on Thursday that any ground campaign against IS would not involve UK troops.
In an interview with political magazine The House, Defence Secretary Michael Fallon said the campaign against IS could be a 'long haul' of 'two to three years'.
The US began a series of air strikes in Iraq last month, and on Monday it began attacks on targets in Syria.
Pentagon spokesman Rear Admiral John Kirby said the purpose of the Syria strikes was 'not necessarily to kill militants' but to destroy the facilities - including oil refineries - which were funding IS through the black market.
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.
IS has threatened to kill British hostage Alan Henning, whose wife pleaded for his release on Tuesday. The threat was made in a video showing the beheading of British aid worker David Haines earlier this month.
Mr Haines's daughter, Bethany, has told ITV News IS must be 'eradicated'.
Also on Tuesday, IS released a second video showing UK journalist John Cantlie.
Meanwhile, in the US, FBI director James Corney says bureau has identified the man referred to in the British press as 'Jihadi John'.
The suspected militant - whom the FBI is not naming at present - is thought to have appeared in IS beheading videos.
Speaking at the UN in New York on Wednesday evening (Thursday morning UK time), Mr Cameron said countries must stop their citizens travelling to join jihadist groups.
He said 500 Britons had gone to join IS.
These apparently included Ibrahim Kamara, 19, from Brighton, whose mother Khadijah told the BBC her son had been killed in a US air strike in Syria on Monday.
She said her son, who had apparently joined al-Qaeda affiliate group Jabhat al-Nusra, had been 'brainwashed'.
Jihadist speaks
Amer Deghayes, from Brighton, said he travelled to Syria with Ibrahim Kamara to join Jabhat al-Nusra.
Speaking to BBC Newsnight from Syria, Mr Deghayes said Mr Kamara was 'soft-hearted' and a 'funny guy' who wanted to help people.
'He was just a normal Muslim lad,' he said.
'He learned about his duty towards the people that are being oppressed, people that are being attacked because they're Muslims and he saw that the solution is jihad.'
Mr Deghayes said he understood why his friend's family were upset, but said Mr Kamara had been granted a 'honour' by Allah.
'Ibrahim has always been a person that asked for martyrdom,' he said.
He said if the family did not 'understand', it was because they did not have 'the knowledge of the virtues of jihad and the virtues of martyrdom'.
Mr Deghayes said he was friends with at least three other British fighters who were killed in Monday's air strikes, but he refused to reveal their identities.
Full text of the government's Commons motion
That this House:Condemns the barbaric acts of ISIL against the peoples of Iraq including the Sunni, Shia, Kurds, Christians and Yazidi and the humanitarian crisis this is causing;
Recognises the clear threat ISIL pose to the territorial integrity of Iraq and the request from the government of Iraq for military support from the international community and the specific request to the UK government for such support;
Further recognises the threat ISIL poses to wider international security and the UK directly through its sponsorship of terrorist attacks and its murder of a British hostage;
Acknowledges the broad coalition contributing to military support of the government of Iraq, including countries throughout the Middle East;
Further acknowledges the request of the government of Iraq for international support to defend itself against the threat ISIL poses to Iraq and its citizens, and the clear legal basis that this provides for action in Iraq;
Notes that this motion does not endorse UK air strikes in Syria as part of this campaign, and any proposal to do so would be subject to a separate vote in Parliament;
Accordingly supports Her Majesty's Government, working with allies, in supporting the government of Iraq in protecting civilians and restoring its territorial integrity, including the use of UK air strikes to support Iraqi, including Kurdish, security forces' efforts against ISIL in Iraq;
Notes that Her Majesty's Government will not deploy UK troops in ground combat operations;
Offers its wholehearted support to the men and women of Her Majesty's armed forces.
Entities 0 Name: Iraq Count: 17 1 Name: Syria Count: 12 2 Name: UK Count: 11 3 Name: US Count: 5 4 Name: ISIL Count: 4 5 Name: Deghayes Count: 3 6 Name: Iraqi Count: 2 7 Name: Ibrahim Kamara Count: 2 8 Name: Brighton Count: 2 9 Name: BBC Count: 2 10 Name: Kamara Count: 2 11 Name: David Cameron Count: 2 12 Name: FBI Count: 2 13 Name: Khadijah Count: 1 14 Name: Yazidi Count: 1 15 Name: Middle East Count: 1 16 Name: Jeremy Corbyn Count: 1 17 Name: Philip Hammond Count: 1 18 Name: United Nations Count: 1 19 Name: Saudi Arabia Count: 1 20 Name: John McDonnell Count: 1 21 Name: John Cantlie Count: 1 22 Name: Ibrahim Count: 1 23 Name: Jabhat al-Nusra Count: 1 24 Name: Liberal Democrats and Labor Count: 1 25 Name: New York Count: 1 26 Name: The House Count: 1 27 Name: Western Count: 1 28 Name: Defense Count: 1 29 Name: Amer Deghayes Count: 1 30 Name: Haines Count: 1 31 Name: British Count: 1 32 Name: Bethany Count: 1 33 Name: Parliament Count: 1 34 Name: John Kirby Count: 1 35 Name: David Haines Count: 1 36 Name: Diane Abbott Count: 1 37 Name: United Arab Emirates Count: 1 38 Name: Shia Count: 1 39 Name: James Corney Count: 1 40 Name: UN Count: 1 41 Name: Alan Henning Count: 1 42 Name: Cameron Count: 1 43 Name: Michael Fallon Count: 1 44 Name: Pentagon Count: 1 45 Name: Sean Curran Count: 1 Related 0 Url: http://ift.tt/1qw5EU6 Title: Iraq crisis: Britain will join air strikes on Islamic State as David Cameron recalls Parliament - Telegraph Description: Isil commander Omar al-Shishani declares the elimination of the border between Iraq and Syria (AP) The Prime Minister used his speech to set out his plans to "destroy" the terrorists, whom he said were a "clear and present danger to the United Kingdom". However, he warned that British involvement in the conflict could last a "long time".
