Britain has a clear "duty" to join air strikes against Islamic State militants because of the direct threat they pose to the country, David Cameron told an emergency Commons session to approve the deployment of UK forces.

The Prime Minister braced the public for a mission that would last "not just months but years" but said he was not prepared to "subcontract" the protection of British streets from terrorism to other countries' air forces.

MPs appear set to vote overwhelmingly in favour of the decision to join the US-led bombing raids against the jihadist forces in Iraq after the escalation was given the support of all three main party leaders.

Labour leader Ed Miliband - who a year ago forced the Prime Minister to abandon plans for air strikes against the Syrian regime by inflicting a Commons revolt on the issue - said the UK "cannot simply stand by".

UK armed forces had "unique assets that no other coalition ally can contribute", such as Brimstone missiles, "which minimise the risk of civilian casualties", Mr Cameron told a chamber packed with MPs recalled from the party conference season recess.

"But I believe it is also our duty to take part," he declared - with RAF jets poised to take to the skies as soon as suitable targets are identified, potentially within hours of the vote.

Britain has six RAF Tornado GR4 fighter bombers stationed within striking range of northern Iraq in Cyprus, a figure which prompted former Cabinet minister Kenneth Clarke to suggest that Britain's military contribution would be "almost symbolic".

Mr Cameron told MPs: "This international operation is about protecting our people too, and protecting the streets of Britain should not be a task that we are prepared to entirely subcontract to other air forces of other countries.

"This is not a threat on the far side of the world."

He pointed to the murder by Islamic State (IS) of hostage David Haines, its threat to kill two other British captives, and a number of terrorist attacks and plots against Britain and other Western nations.

"Left unchecked, we will face a terrorist caliphate on the shores of the Mediterranean, bordering a Nato member, with a declared and proven determination to attack our country and our people.

"This is not the stuff of fantasy - it is happening in front of us and we need to face up to it."

Warning of a lengthy engagement, he said: "This is going to be a mission that will take not just months but years, but I believe we have to be prepared for that commitment."

He added, in a reference to the 2003 US-led invasion which many critics cite as a reason for opposing the latest strikes: "The hallmarks of this campaign will be patience and persistence, not shock and awe.

"We must not use past mistakes as an excuse for indifference or inaction."

He insisted he believed 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. MPs on both sides of the House warned that failure to act in Syria would leave a bolthole for IS to escape Allied attacks in northern Iraq.

Mr Miliband has called for any such extension - which would require a further Commons vote - to be backed by a United Nations Security Council resolution.

Mr Cameron said he did not believe there was any "legal barrier" but acknowledged the situation there was "more complicated" and that there was currently no consensus in the Commons for action in Syria.

But he stressed that he retained the right to take military action without prior parliamentary approval to prevent an imminent humanitarian catastrophe or urgent threat to national security.

The Archbishop of Canterbury, the Most Rev Justin Welby, backed the air strikes, telling the House of Lords: "The action proposed today is right."

But he warned "we must not rely on a short-term solution" and a wider effort was needed to turn extremists away from the "evil of Isil".

The widow of the murdered British hostage, Dragana Prodanovic Haines, said she backed UK air strikes "limited to distinctive targets".

"No changes in such societies can be enforced, they have to come from within," she told Sky News.