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 Islamic state targets in Raqqa, the group’s stronghold, and other areas.
While 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 Mr Corbyn and others he described as “a bunch of terrorist sympathisers”.
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.
The UK is already providing intelligence, surveillance and other logistical support to countries fighting Islamic State in Syria and the RAF has carried out thousands of raids on IS targets in Iraq since Parliament approved similar action there last year.