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”.