Wary British MPs are expected to back military action in Iraq, as part of a daunting mission to destroy Islamic State (Isis) that the defence secretary Michael Fallon said could endure for as long as two to 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 and specifically excludes air strikes in Syria without a further Commons vote.

Fallon said the war would be “a long haul” and 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 that are being 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.”

Before the recall, MPs from all sides were expressing concern about the dangers of mission creep and the absence of identifiable or achievable war aims in the Commons motion that describes the war almost exclusively in terms of answering a request by the Iraqi government to defend its territorial integrity.

Despite that, whips from the three main parties have told their leaders that they will comfortably win the vote to authorise British involvement in air strikes against targets from Islamic State.

That will come in marked contrast to a year ago when David Cameron unexpectedly lost a Commons vote on air strikes designed to punish President Bashar al-Assad of Syria last summer. The prime minister has worked painstakingly to ensure Labour support in advance of Friday’s debate with a carefully crafted motion that is precise about what it authorises.

As a result, in debate 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. But 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 foreign 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.

“If your answer is to start bombing Isis in Iraq then what you are actually doing is bombing the tribes and the Ba’athists who are exactly the people you are going to need to get rid of Isis. It is a political problem.” He added: “I am not going to vote for a bombing campaign that hasn’t been thought through.”

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.”

He continued: “John Kerry [the US secretary of state] 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.”

Fallon sought to justify the action by saying: “We all have a very direct interest. Britain does above all. We’ve seen already terrorist attacks here – the London tube, London buses, the murder of Lee Rigby, the attack on Glasgow airport – we’ve already been under attack from this kind of extremism and we have to deal with it. This is a very direct British interest and our armed forces are ready for it.”

The daughter of David Haines, the British aid worker beheaded by the militant organisation, told ITV News that she believed Britain needed to get involved in air strikes. “IS need to be eradicated. They can’t continue this way. 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.”

Fallon said any action in Syria would have to be separately endorsed by parliament, but clearly thought action in Syria could follow Iraq. He explained: “They are different countries. Everything we’ve done in Iraq so far – every flight, every delivery – has been with the permission or at the invitation of the Iraqi government. Everything we are doing in Iraq we are authorised to do.

“And that doesn’t apply to Syria, so it’s a different legal situation and it’s also a different military situation because of Syria’s quite formidable air defence system.”

A summary of the legal advice provided to cabinet by Attorney General Jeremy Wright repeatedly highlighted states the prohibition of the use of force “does not apply to the use of military force by one state on the territory of the other if the territorial state so requests or consents.” He added that it was clear in this case that Iraq has consented to the use of military force to defend itself against Isis in Iraq.