Britain is prepared to “make the case” to extend air strikes against Islamic State (Isis) beyond Iraq into Syria if it believes the circumstances are right, the foreign secretary, Philip Hammond, has said.

As wary British MPs return to Westminster to vote on whether to back military action in Iraq, Hammond said the government believes there is a robust legal basis for extending the strikes into Syria.

Hammond spoke as the defence secretary, Michael Fallon, warned that Britain and its allies are facing a daunting mission to destroy Isis that could endure for as long as three years. The Commons motion, agreed unanimously by the cabinet and due to be debated by MPs for seven hours, is couched in terms of protecting Iraq from the brutality of Isis, also known as Isil, and specifically excludes air strikes in Syria without a further Commons vote.

The foreign secretary said Britain had no plans to extend the strikes beyond Iraq but said its plans could change. Asked on the Today programme on BBC Radio 4 whether Britain could extend its air strikes into Syria, Hammond said: “We would look at the circumstances at the time. If we felt we had some capability to contribute that was needed in order to deliver the outcome that we all want to see, which is the defeat of Isil and its poisonous ideology, then we would certainly make the case for doing that if the circumstances were right.”

Hammond said extending the air strikes into Syria would require a second parliamentary vote. But he indicated that the legal advice used by the US to underpin its air strikes in Syria – the declaration by the Iraqi government which is necessary on the grounds of “collective self defence” – could be used by Britain.

The foreign secretary said: “At the moment the government of Iraq has asked us to intervene in Iraq. It is clear that the US intervening in Syria is also able to do so on a legal basis of collective self defence because the government of Iraq has identified that there is a present threat to Iraq coming from Syria. We haven’t looked at this in terms of our own permissions ... because we are not proposing to do it. That is the legal basis that the US is relying on. It looks robust to me.”

Fallon, who succeeded Hammond as defence secretary, said the war would be “a long haul” as he made little attempt to deny that Isis’s destruction may eventually, subject to another vote from MPs, require coalition action against Isis militants in Syria. The motion, the product of extensive negotiations with the Labour leader, Ed Miliband, also promises that the UK will not deploy troops in ground combat operations in Iraq, a wording that nevertheless permits the presence of trainers, intelligence and special forces to help guide RAF jets and strengthen Iraqi and Kurdish forces.

The motion asks MPs to support “the government of Iraq in protecting civilians and restoring its territorial integrity” by measures “including the use of UK air strikes to support Iraqi, including Kurdish, security forces efforts against Isil in Iraq”.

The cabinet, handed clear advice from the attorney general justifying the war’s legality, was also briefed by the home secretary, Theresa May, on the threat of reprisals by Isis supporters in the UK.

It is expected that six RAF Tornado jets will be in action within as little as 24 hours of the vote. The jets, based in Cyprus, were out over Iraq on Wednesday night gathering intelligence in preparation for attacks co-ordinated with the US-led coalition, as well as Iraqi and Kurdish armed forces.

After the initial strikes it is likely there will be a pause as the RAF identify further Isis targets for attack. A similar pause occurred after initial French air strikes. A British source said: “This is not about hundreds of bunkers that will be targeted straight away.”

Whips from the three main parties have told their leaders they will comfortably win the vote to authorise British involvement in air strikes against Isis. A modest but noticeable number of MPs on both sides will rebel against the three-line whip imposed by all the party leaders calling on them to support air strikes against Isis forces in Iraq.

Diane Abbott, Labour’s former shadow public health minister, said she would vote against the action. “We have all been horrified by the atrocities committed by Isil and the images of hostages about to be beheaded. But the truth is a lot of MPs have misgivings about this. We think we’ve seen this movie and we know how it ends.”

Nigel Farage, the Ukip leader, who complained the prime minister had recalled parliament to overshadow the opening of his party’s annual conference in Doncaster, said he opposed the military strikes. Farage told the Today programme: “I would vote against. I don’t see that us just playing catch up with the Americans – and launching a bombing raid – is actually going to work. 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.

“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.”

While all party leaders face rebellions on their benches, the whips are predicting the most notable element of the debate is likely to be heartfelt speeches of concern from loyalists and rebels. Keith Simpson, who served as William Hague’s parliamentary private secretary during his four years as home secretary, told the Guardian: “A number of colleagues will, either in their speeches during the debate or in the form of interventions, raise serious questions because they do have concerns about the overall coalition strategy, about means and ends, whether you can continue air strikes just on Iraq targets or whether pressure will be brought to bear on us to include targets in Syria.”

Adam Holloway, the Conservative MP for Gravesham, who served with the Grenadier Guards during the first Gulf war, told the Guardian: “It has just not been thought through. This is a political problem. Isis in Iraq are Sunni tribesmen who were fed up with the Maliki government. They are international jihadis and they are former Ba’ath regime elements. The only way you are going to get rid of the foreign jihadis is if the Sunni tribes and the Ba’athists do it themselves.”

Fallon, who was accompanied at the cabinet by the chief of the defence staff, General Sir Nick Houghton, and senior figures in the intelligence agencies, made no attempt to pretend that Britain’s third military intervention in Iraq since 1990 would be swift or easy.

The defence secretary said: “This has to be planned, it has to be sustained. This is going to be a long drawn-out campaign which we have to be careful, methodical and measured about.

“But equally there’s a determination right across Nato to tackle Isil. Because if we don’t, it comes back on us.”