Islamic State (IS) is a threat to the "streets of Britain" and the UK has a "duty" to confront it, David Cameron has said as MPs debate air strikes.
The prime minister said it was in the UK's interest to join in bombings against IS in Iraq and there was also a strong case for action in Syria.
Parliament is due to vote on UK involvement in Iraq at about 17:00 BST.
Downing Street has said a small number of troops could be sent to Iraq within hours if the Commons backs action.
However, the prime minister's official spokesman emphasised the troops would not be in a combat role but would be used to guide air strikes, in a humanitarian role and, possibly, to train Iraqi and Kurdish peshmerga forces, although this may take place in neighbouring countries.
Syria next?
The government is expected to win the vote comfortably, with the Conservatives, Liberal Democrats and Labour all supporting the action after it was requested by the Iraqi government.
Mr Cameron told MPs there was a "strong case" for the UK to do more in Syria, where America and Arab allies are engaged in aerial bombardment of IS positions.
The PM said he did not believe there was any "legal barrier" to military action there - but he acknowledged there was no consensus for such a move among MPs.
Downing Street said there would be no action in Syria without a Commons vote unless there is an urgent humanitarian need to do so.
In other developments:
- The Pentagon said four IS tanks had been destroyed during the latest raids in Syria
- Labour leader Ed Miliband said IS "cannot and should not be negotiated with".
- The widow and daughter of murdered British aid worker David Haines back RAF air strikes
- Speaking in the House of Lords, the Archbishop of Canterbury, Justin Welby, supported action but called for "an ideological and religious" solution rather than a military one.
- Denmark is to contribute seven planes to the international coalition against IS
- The number of Europeans joining Islamist fighters in Syria and Iraq has risen to more than 3,000, the EU's anti-terrorism chief says
IS has seized large parts of Iraq and Syria in recent months. The group, also known as Isis or Isil, has used tactics that have included beheadings of soldiers, Western journalists and aid workers.
Making the case for British action, Mr Cameron said the killing of a British hostage illustrated that the challenge was not "on the far side of the world" and the "brutal, terrorist organisation" was a "clear and proven" threat to UK lives.
"This is not the stuff of fantasy. It is happening in front of us and we need to face up to it," he said.
'Not fantasy'
Citing attacks by IS on targets in Europe, and the growing number of foiled terror plots, he said the organisation "already declared war on us and there was no walk on-by option".
Stressing that the UK had "unique assets" to contribute to the military offensive, he added: "It is our duty to take part.
"This international operation is about protecting our people, too, and protecting the streets of Britain."
Analysis by Chris Mason
It's the biggest question any prime minister can be asked.
It's the biggest decision any parliament ever takes.
Should our armed forces be sent into combat?
There is likely to be an overwhelming majority in favour of military action against IS in Iraq. But many MPs are asking another question: what next?
And so many are saying, yes to today's question but: Will it be Syria next?
At what cost? And when will this end?
Addressing concerns from MPs that the UK could be dragged into a long war, Mr Cameron said the use of combat troops would be "wrong" and conceded air strikes would not "roll back" IS alone but must be part of "comprehensive" political and a humanitarian plan.
Referring to previous external interventions in Iraq, he added: "This is not 2003 but we should not use past mistakes as an excuse for inaction or indifference."
Backing Mr Cameron on Iraq, Labour leader Ed Miliband said IS was a "murderous" organisation intent on "more killing" although he urged the endorsement of the United Nations for military intervention.
"There is no graver decision for our Parliament and our country. But protecting our national interest, security and the values for which we stand is why I will be supporting the motion this afternoon."
On Thursday, about 250 people protested outside the gates of Downing Street against the possibility of military action in Iraq.
Some MPs also questioned the objectives behind the military action.
Conservative backbencher Edward Leigh said air strikes could be seen as "gesture politics", Labour's Denis Skinner warned of "mission creep" while Green Party MP Caroline Lucas said "killing extremists does not kill their ideas".
And Respect MP George Galloway said IS was more of a "death cult" than a recognised army and, if there was to be bombing, it should be left to regional powers such as Saudi Arabia and Turkey.
"The last people who should be returning to the scene of their crimes are the UK, US and France", he said.
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.
Analysis
by Jonathan Beale, defence correspondent
Six RAF Tornados in Cyprus have been ready to carry out air strikes for weeks. Their laser-guided bombs and missiles will be loaded and armed soon after Parliament gives the green light.
The Tornados have already been flying reconnaissance missions over Iraq and will have identified potential targets. But some of the obvious ones, like command and control centres, will have already been hit by US war planes which have been launching strikes for the past month.
The focus of their attacks have now switched to Syria itself. So the RAF will also be looking for targets of opportunity - such as IS fighters and vehicles on the move.
The expected intervention of the UK will not be a game-changer. During military intervention against Libya in 2011, the RAF had three times as many war planes involved. But the fight against IS will be more like a marathon than a sprint.
And the longer it goes on, IS will adapt its tactics, and air strikes will inevitably become less potent.
Black market
The government's motion specifically rules out any attacks on IS in Syria, following the Commons' rejection of joining in air strikes in that country last year.
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.
The US began a series of air strikes in Iraq last month, and on Monday it began attacks on targets 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.
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. Also on Tuesday, IS released a second video showing UK journalist John Cantlie.
Meanwhile, in the US, FBI director James Corney says the bureau has identified the man referred to in the British press as "Jihadi John".
BBC security correspondent Gordon Corera said UK security services were frustrated at the FBI comments.
He said UK authorities "may well" have known the suspect's identity for weeks, but they wanted to gather as much intelligence as possible without "tipping their hand as to what they knew".
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, Mr Cameron said countries must stop their citizens travelling to join jihadist groups.
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".
