The British air force could conduct airstrikes in Iraq as early as Friday night after parliamentarians approved the intervention in an emergency debate — but any action in Syria was kept firmly off the table.

Airstrikes were approved by an overwhelming majority of 524 to 43 following a seven-hour debate which saw passionate scenes in the House of Commons as parliamentarians took to the floor one by one to voice their demands for — or opposition to — intervention against the Islamic State.

But British planes will not be allowed to join strikes across the border in Syria, a decision dismissed by defence specialists as a half-measure in the fight to stop a caliphate being established across the two countries.

Prime Minister David Cameron said Britain had never dealt with a threat like the Islamic State before, as he opened the debate on Friday morning. "This is not 2003," he insisted, alluding to the intervention against Saddam Hussein.

"This is not a threat on the far side of the world, left unchecked we will face a terrorist caliphate on the shores of the Mediterranean ... This is not a fantasy and we need to face up to it," he told members of parliament (MPs).

Cameron laid the failure to consider strikes in Syria firmly at the door of the opposition, saying there was a "strong case" for such a move but that he had not put it to parliament at this stage because of the need for consensus. But he acknowledged British concerns over the delicacy of intervention in the multi-dimensional Syrian conflict, noting that the situation in the country was more complicated because of the "brutal dictator" Bashar al-Assad.

Britain's politicians have been criticised for being late to the game over the conflict in Iraq and Syria, while others such as the United States, France and a number of Gulf states take on the militants bidding to carve out an Islamic caliphate in the Middle East.

Amid a growing clamour for intervention, all three major parties now support British involvement in airstrikes — but the Labor opposition has insisted that a United Nations resolution would be required for it to back intervention in Syria. Some Labor MPs remain resolutely opposed to any intervention at all, and one — Rushanara Ali, MP for Bethnal Green and Bow — resigned her post in the opposition shadow cabinet ahead of the vote in order to abstain.

Parliamentarians are still stung by the memory of the 2003 Iraq invasion and have been reticent ever since to sanction military action in the Middle East. Last year Prime Minister David Cameron suffered a humiliating defeat when parliament voted against intervention in Syria following chemical weapons attacks on civilians, to then be snubbed by Washington as it embraced France as its "oldest ally."

On Thursday night protesters gathered out Downing Street to voice their opposition to airstrikes. "Don't bomb Iraq!" read placards from the Stop the War Coalition.

But the demonstration belied the position of the British public, which now overwhelmingly supports airstrikes not only in Iraq but in Syria too, according to opinion polls.

A YouGov poll for The Sun newspaper published on Friday indicated that 57 percent back British strikes in Iraq with just 24 per cent opposed — a 20 percent rise in support since August. 51 percent are in favour of extending action to Syria, with 26 percent against.

The public also seems to have regained its appetite for conflict in the wake of the Iraq and Afghanistan wars in a way that politicians have not — 43 percent would like to see British troops back in Iraq or to at least keep the option on the table. Only 32 percent want to completely rule out boots on the ground.

In an attempt to win over reluctant MPs from his party, Labor leader Ed Miliband insisted that this was not a repeat of 2003 and was aimed at supporting a democratic state rather than regime change.

"There is no debate about the legal case as in 2003. Whatever side of this debate you're on, nobody is saying 'Let's negotiate with ISIL (Islamic State)'. There is broader support… five Arab states taking part in action," he said.

There was nevertheless vehement opposition to the move from some anti-interventionist stalwarts. George Galloway, leader of the fringe left Respect Party, insisted that "everything will be made worse" by strikes, arguing that the populations of Iraq and Syria were to an extent supportive of IS because of western policies.

"The last people who should be returning to the scene of their former crimes are Britain, France and America," he added.

But defence specialists insisted that action, while challenging and likely costly, was essential - and that it would inevitably have to be extended to Syria.

Lord Dannatt, the former head of the army, said attacking the Islamic State in Iraq only was "dealing with half a problem, not a whole problem."

He acknowledged that operating in Syrian airspace was a problem and might necessitate some form of dialogue with the Assad regime. In the end, he suggested, boots on the ground might be inevitable.

"Issues such as the one we are currently facing are ultimately settled on the ground. Isil must be defeated on the ground, albeit supported from the air," Dannatt said.

Defense Secretary Michael Fallon told BBC Radio 4 on Friday that the Islamic State's headquarters were in Syria, not Iraq, and that the group could not be defeated without striking at its stronghold.

He said: "ISIL is based in Syria, that's where its headquarters, resources and people are. They have crossed the border into Iraq, and to deal with ISIL you have to deal with and defeat them in both Iraq and Syria. The logic follows."