Foreign Secretary Philip Hammond has warned that Britain has "no choice" about tackling the threat of Islamic State (IS), as MPs prepare to vote on whether to join air strikes in Iraq.

The minister said the extremist group posed a clear threat to UK national security.

He also reiterated that the bombing campaign could be extended to Syria in future - although he stressed that Parliament would be consulted again before that happened.

The Cabinet unanimously backed military action against IS yesterday, and the Government is expected to secure a large majority in Parliament later.

Meanwhile, the head of the FBI said it has identified the British extremist known as "Jihadi John" who was shown beheading hostages in three gruesome videos.

Mr Hammond refused to speculate on how long the military campaign against IS could last, after Defence Secretary Michael Fallon suggested it could be up to three years.

"It will take as long as it takes. I don't think anybody can put a timeframe on it," Mr Hammond said.

"We are going into this with our eyes open. We don't have a choice about engaging with Isil ... it is a threat to Britain's national security, it is a threat to the stability of the region."

Mr Hammond said the Iraqi army needed "restructuring and retraining" after being "damaged" by Nouri Maliki's government.

But he stressed that the force on the ground was large and "well-equipped", and only required air and intelligence support to defeat IS.

Mr Hammond again dismissed the idea that Britain could strike a deal with Syrian president Bashar Assad.

"Assad has put himself beyond the pale. We are not co-operating with or working with or negotiating with Assad at all," he said.

Amid criticism from some MPs that the threat from IS cannot be addressed without striking the territory it holds in Syria, Mr Hammond denied that the British approach was "half-hearted".

He said that the strategy went much wider than "military action alone".

The motion being debated by the Commons states that troops will not be deployed in "ground combat operations" against IS - also known as Isil (Islamic State of Iraq and Levant).

It also makes clear that another vote will be called if the air campaign is to be extended to Syria.

A summary of the Government's legal position released alongside the motion argues that the action is permissible because Iraq has requested military help to protect its "people and territory" - without making any reference to the threat IS poses to the UK.

Labour backbencher Diane Abbott told Today the decision was more "complex" than last year, when the Commons refused to back air strikes in Syria.

But she added: "The fact remains, we're in for the long haul with this, we're going to be spending billions of pounds.

"I think that the military action on its own is going to be no more successful in resolving things in Iraq than the original Iraq war was."

Ukip leader Nigel Farage said he would vote against the motion if he were in Parliament.

"I suspect that it will kill a lot of civilians, which a few weeks down the road will lead us to wonder why we started this," he said.

"I also don't believe it when the Prime Minister says it will make the streets of Britain safer. I suspect the opposite may be true."

The daughter of one of the murdered hostages, British aid worker David Haines, urged politicians last night to press ahead with strikes to "eradicate" the extremists.

Bethany Haines said: "They can't be doing this to people and get away with it. No matter what nationality, if they are Western or not. Hundreds of Syrians have been killed by them. They need to be stopped. If air strikes and ground force is what it takes, that's what it takes."

Speaking at a briefing with US journalists yesterday, FBI director James Comey said he knew the identity of the jihadi with a London accent who has so far featured in videos showing the murder of American journalists James Foley and Steven Sotloff, as well as Mr Haines.

The man is said to lead a small group of British jihadists who became known by hostages as "The Beatles".

"I believe that we have identified him. I'm not going to tell you who I believe it is," Mr Comey reportedly said.

Asked if he would prioritise capturing the killer, he said: "We will do, and expend the effort that I think the American people would want us to and expect us to."

The Home Office said it did not comment on security operations.

If agreed by Parliament, the UK would expect to join the US-led international air campaign within days. But the bombing may not prove that intense as IS is understood to have limited numbers of fixed positions that can be attacked.

Mr Fallon has suggested that the campaign against IS could be a "long haul" of "two to three years".

He said: "(US secretary of state) John Kerry has estimated two to three years, that looks like a long haul to me. But we have to face up to this. This kind of extremism has been spreading, taking root in democracies."

Mr Cameron has deliberately restricted Britain's involvement to Iraq, in part to secure the support of Labour, which has raised concerns about extending air raids into Syria without specific authorisation of the UN Security Council.

The Prime Minister is desperate to avoid a repeat of last year's Commons vote on military action when Labour combined with Tory rebels to inflict a damaging defeat on the Government.

Unlike Iraq - where Britain is acting at the request of the government in Baghdad - there has been no such request from the Syrian regime of President Assad, raising questions over the the legality of any military intervention.

But a YouGov poll suggested public opinion backed air strikes in Syria as well as Iraq.

The poll showed that 57% of UK residents surveyed say they supported targeting IS in Iraq, up 20% from a similar poll last month, compared to 24% who were against the move, while 51% gave their backing to strikes on IS positions in Syria, against 26% who said no.

And some 43% said they would either want to see the US and UK send in ground troops to Iraq, or at least keep the option open, while 32% disagreed.