MPs will decide later whether to back UK air strikes in Syria against militants from so-called Islamic State.
A 10-hour House of Commons debate will culminate in a vote on whether the UK should join the US, France, Russia and others bombing targets in Raqqa, the group’s stronghold, and other areas.
UK Prime Minister David Cameron says IS is a threat to Britain’s security.
Labour leader Jeremy Corbyn opposes bombing but has given MPs a free vote amid divisions within his own ranks.
With up to 50 Labour MPs likely to back the government – and both the Democratic Unionist Party and the Liberal Democrats backing action – Mr Cameron is expected to win parliamentary approval for the UK to intervene militarily in the four-year conflict in Syria.
However, at least 110 MPs from six different parties – including the SNP, which opposes action – have already signed up to an amendment seeking to block air strikes.
The prime minister caused controversy on the eve of the vote by labelling Mr Corbyn and other opponents of action as “terrorist sympathisers”.
The BBC’s political editor Laura Kuenssberg said the comments were a departure from the “carefully crafted” language that Mr Cameron has used over the past week.
It comes amid reports that Downing Street – which has been trying to court Labour MPs – was now keen to carry the vote through with support of its own MPs and the party’s “natural allies”, such as the DUP.
A spokesman for Mr Corbyn said Mr Cameron’s comments were a “contemptible slur” and showed he was losing the argument.
Addressing a meeting of the 1922 Conservative backbench committee, Mr Cameron warned that if Tory MPs voted against strikes they risked undermining a strong message that the UK was standing alongside its allies already engaged in military action.
According to BBC research, of the 640 MPs expected to vote, 362 MPs are in favour of the motion while 175 are against. Of the remainder, 19 are “leaning to” supporting the government, three are “leaning against” while 80 are undecided.
Wednesday’s parliamentary schedule – including Prime Minister’s Questions – has been scrapped to accommodate a whole day’s debate on Syria.
The government says military action is “only one component of a broader strategy” to tackle IS and the UK government would not deploy troops on the ground.
The prime minister is likely to face tough questions about the scope of air strikes, their likely impact and how they fit into the strategy of helping to stabilise and rebuild Syria.
Mr Cameron has been asked to explain his claim there are 70,000 “moderate” ground forces able to fight IS in Syria.
The UK is already providing intelligence, surveillance and other logistical support to countries fighting IS in Syria. The RAF has also carried out thousands of raids on IS targets in Iraq since Parliament approved similar action there last year.
Some nights in Westminster you can feel a crackle in the air, you can sense the tension, and sometimes even see the weight of responsibilities that MPs know they carry collectively in the looks on their faces. Last night was one of them.
For government ministers this vote has been a very long time coming, an obvious extension of the action British forces are taking in Iraq.
Bombing is, to them, part of a complicated set of solutions, but a straightforward decision to make. Don’t confuse that however with a sentiment that it is an easy choice.
The run-up to the vote has been marked by a week of turmoil within the Labour Party.
While Mr Corbyn has the support of the majority of his MPs, up to half of his shadow cabinet may vote in favour of bombing, including Shadow Foreign Secretary Hilary Benn and Deputy Leader Tom Watson.
The Labour leader urged those who take a different view to him to “think again”, saying 75% of Labour members polled by the party indicated they opposed air strikes.
Party sources have claimed the number of Labour MPs likely to back the government is falling.
Mr Corbyn said efforts should focus on a political settlement and achieving a “credible line of government” across Syria.
SNP leader Nicola Sturgeon has confirmed her party’s 54 MPs will be opposing air strikes, saying bombing on its own will not rid the threat of terrorism or bring peace to Syria.
But the DUP has said its eight MPs will support airstrikes and Lib Dem leader Tim Farron has said action is justified as part of “a measured, legal and broad-based international effort”.
MPs rejected air strikes against Syrian government targets in 2013, but ministers say it is “illogical” to carry out strikes in Iraq but not Syria – as IS does not recognise the international border.
Thousands of protesters, led by the Stop the War coalition, took to the streets of London for the second time in four days on Tuesday to protest against bombing.(BBC News)
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