MPs will decide later whether to back UK air strikes 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, Cameron is expected to win parliamentary approval for the UK to intervene militarily in the four-year conflict in Syria.