LONDON - Prime Minister David Cameron warned that British military action against Islamic State militants could last for "years" as he urged MPs on Friday to vote in favour of joining US-led air strikes in Iraq.
Cameron kicked off an emergency House of Commons debate with a call to action against the "psychopathic terrorists" who have beheaded British aid worker David Haines and are holding two more Britons, Alan Henning and John Cantlie.
"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," the prime minister said.
"We should not expect this to happen quickly. The hallmarks of this campaign will be patience and persistence, not shock and awe."
While MPs are expected to approve the move, many questioned why Britain was not also joining military strikes on Syria.
The debate has also evoked memories of Britain's role in the deeply unpopular US-led invasion of Iraq in 2003 under Labour prime minister Tony Blair, which led to the death of 179 British personnel over six years.
Six British Tornado fighter jets based in Cyprus are poised to begin raids on IS militants within days or even hours after the vote, due at 5:00 pm (1600 GMT), is passed.
Britain would join the United States and France in launching targeted strikes on IS jihadists in Iraq, where they control swathes of territory, as in neighbouring Syria. Denmark, Belgium and the Netherlands are also poised to take part.
It will not at this stage join US-led air strikes on Syria, and Cameron said a separate vote would be needed before that could happen.
That option is not currently on the table due to a lack of consensus between Britain's main parties which would likely see the proposal defeated.
The opposition Labour party, which inflicted a humiliating defeat on Cameron last year over military action against Syrian President Bashar al-Assad, has indicated it would require a UN Security Council resolution before backing action in Syria.
However, all the main parties are backing Friday's motion on air strikes in Iraq, meaning that it is almost certain to be approved.
As well as atrocities committed in Iraq itself, Cameron argued that taking action was a matter of protecting Britain's streets which should "not be a task that we are prepared to entirely subcontract to other air forces".
On Thursday, police in London arrested nine people suspected of extremist Islamist links and another two were held on Friday.
But MPs who had returned to London for an emergency recall of parliament repeatedly questioned the scope of the mission.
Peter Hain, a former senior member of Blair's Labour government, said the lack of British action over Syria was "the elephant in the room".
"Simply allowing ISIL (another term for the IS group) to retreat across an invisible border, which they control, into Syria and regroup is simply no answer," he said.
And Richard Ottaway, chairman of the House of Commons foreign affairs committee and a member of Cameron's Conservative party, warned of likely British deaths in a "messy" conflict.
A small number of MPs, for whom the 2003 Iraq invasion remains a painful memory, are expected to oppose military action.
Veteran firebrand George Galloway said the IS militants were a "death cult" and would not be destroyed by bombings.
"The last people who should be returning to the scene of their former crimes are Britain, France and the United States of America," he said.
The Stop the War Coalition -- which helped organise a million-strong demonstration against the 2003 Iraq war -- staged a protest of around 200 people Thursday and has vowed further demonstrations if the vote passes.
In a parallel debate in the House of Lords, Archbishop of Canterbury Justin Welby also backed the air strikes, describing them as "right".
Meanwhile, a former head of the Royal Air Force, Air Chief Marshal Michael Graydon, warned that a lengthy campaign would be "quite a stretch" due defence budget cuts, around eight percent in the four years to 2014-15. (AFP)