MPs will vote later on whether the UK
should join US-led air strikes on Islamic State (IS) in Iraq.
The Tories, Liberal Democrats and Labour all back action, which the coalition
says is legal because it was requested by the Iraqi government.
In an interview with political magazine
The
House, Defence Secretary Michael Fallon said the campaign could be a "long
haul" of "two to three years".
IS has seized large parts of Iraq and Syria in recent months.
The group, also known as Isis or Isil, has used tactics that have included
beheadings of soldiers, Western journalists and aid workers.
It could have as many as 31,000 fighters in Iraq and Syria,
according to the
CIA.
The UK government is not proposing any involvement in air strikes on
Syria.
On Thursday, about 250 people protested outside the gates of Downing Street
against the possibility of military action in Iraq.
Some MPs also oppose the military action.
Labour's Diane Abbott, Jeremy Corbyn and John McDonnell signed a
letter
to the Guardian which said bombing Iraq would "only exacerbate the
situation".
"We urge the government to rule out any further military action in Iraq or
Syria," they said.
The Commons has been recalled from its four-week break for the Scottish
independence referendum and party conferences to deal with the developing
situation.
Prime Minister David Cameron will open the debate at 10:30 BST (09:30 GMT) -
it is expected to last seven hours.
The government does not have to seek the approval of MPs to commence military
action, but it has become customary to do so since this first happened over the
Iraq war in 2003.
Analysis
by Jonathan Beale, defence correspondent
Six RAF Tornados in Cyprus have been ready to carry out air strikes for
weeks. Their laser-guided bombs and missiles will be loaded and armed soon after
Parliament gives the green light.
The Tornados have already been flying reconnaissance missions over Iraq and
will have identified potential targets. But some of the obvious ones, like
command and control centres, will have already been hit by US war planes which
have been launching strikes for the past month.
The focus of their attacks have now switched to Syria itself. So the RAF will
also be looking for targets of opportunity - such as IS fighters and vehicles on
the move.
The expected intervention of the UK will not be a game-changer. During
military intervention against Libya in 2011, the RAF had three times as many war
planes involved. But the fight against IS will be more like a marathon than a
sprint.
And the longer it goes on, IS will adapt its tactics, and airstrikes will
inevitably become less potent.
Black market
Its
motion specifically rules out any attacks on IS in Syria, following the
Commons' rejection of joining in air strikes in that country last year. Any
action there would require a separate vote.
Downing Street said UK forces could join the bombing of targets in Iraq after
the Iraqi minister of foreign affairs wrote to the United Nations seeking
international assistance.
Speaking on BBC Radio 4's Today programme, Foreign Secretary Philip Hammond
said: "The plan is to provide air support to the Iraqi government, alongside our
allies - the US, France and an increasing number of Arab countries who are going
to take part."
On Thursday, Mr Hammond had reiterated that any ground campaign against IS
would not involve UK troops.
The US began a series of air strikes in Iraq last month, and on Monday it
began attacks on targets in Syria.
Pentagon spokesman Rear Admiral John Kirby said the purpose of the Syria
strikes was "not necessarily to kill militants" but to destroy the facilities -
including oil refineries - which were funding IS through the black market.
Jets from Saudi Arabia and the United Arab Emirates have joined US forces in
the attacks, and the US says more than 40 countries have offered to join the
anti-IS coalition. BBC