MPs will decide later whether to back UK air trikes in Syria directed 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 also giving their backing, Mr Cameron is expected to win parliamentary approval for the UK to intervene militarily in the four-year conflict in Syria.
The prime minister caused controversy on the eve of the vote by urging Tory MPs not to “sit on their hands” or “walk through the lobbies” with Corbyn and others he described as “a bunch of terrorist sympathisers”.
Addressing a meeting of the 1922 Conservative backbench committee, Cameron warned that if Tory MPs voted against airstrikes they risked undermining a strong message that the UK was standing alongside its allies already engaged in military action.
The BBC’s political editor Laura Kuenssberg said the comments were a departure from the “carefully crafted” language that Cameron has used on Syria over the past week.
They come amid reports that Downing Street – which has been trying to court Labour MPs – was keen to carry the vote through a combination of its own MPs and the party’s “natural allies”, such as the DUP.
A spokesman for Corbyn said they were a “contemptible slur” and showed the prime minister was losing the argument.
The UK is already providing intelligence, surveillance and other logistical support to countries fighting Islamic State – also known as ISIL or Daesh – in Syria and the RAF has carried out thousands of raids on IS targets in Iraq since Parliament approved similar action there last year.
While the vast majority of Conservative MPs support the extension of airstrikes to Syria, the prime minister is likely to face tough questions about their scope, likely impact and how they fit into the “broader” political and diplomatic strategy the UK has promised to pursue to help stabilise and rebuild Syria.
In particular, Cameron has been asked to explain his claim there are 70,000 moderate ground forces able to fight IS in Syria.
The run-up to Wednesday’s vote, which comes less than three weeks after 130 people were killed in a series of terror attacks in Paris, has been marked by a week of turmoil within Labour.
While 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 has urged those who take a different view to him to “think again”, arguing that they must take account of public opinion and that 75% of Labour members polled by the party indicated they were opposed to air strikes.
BBC political correspondent Carole Walker said 110 MPs of all parties had signed their support for a motion opposing the government plans for airstrikes. Its backers believe they have a strong chance of forcing a vote on the motion, she added.
Wednesday’s normal parliamentary schedule, including prime minister’s questions, has been scrapped to accommodate a whole day’s debate on Syria, beginning at 11:30 GMT. A vote is expected some time around 22:00 GMT.
The government motion to be voted on would authorise air strikes “exclusively” against IS in Syria. It says military action is “only one component of a broader strategy” to tackle IS and the UK government would not deploy troops in “ground combat operations”.
Anti-war protesters took to the streets of London on Tuesday to voice their opposition to bombing targets in Syria
Labour’s leader has said bombing is not a sensible way to bring peace to Syria and party sources have claimed the number of their MPs likely to back the government is falling as the vote approaches.
Speaking on Tuesday, Corbyn warned against the UK being drawn into another conflict in the Middle East and said Labour MPs should heed “what the implications are for this country”.
Rather than air strikes, 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.