David Cameron is making the case for air strikes against IS in Syria, saying the aim is to “keep the British people safe” from terror attacks.
The 10 hour Commons debate will end with a vote on whether the UK joins others such as France, the US and Russia in bombing targets in Syria.
The government motion would authorise air strikes “exclusively” against Islamic State - also known as Isis, Isil or Daesh - in Syria.
Mr Cameron said: “The question before the House on Wednesday is how we keep the British people safe from the threat posed by Isil.
“This is not about whether we want to fight terrorism, it’s about how best we do that.”
Mr Cameron said that in future the UK government would be referring to IS as Daesh as much as possible, because “this evil death cult is neither a true representation of Islam nor is it a state”.
Daesh has negative connotations in the Middle East and is seen by some as a way of challenging the legitimacy of the group.
Labour leader Jeremy Corbyn opposes bombing but has given MPs a free vote amid divisions within his own ranks.
Mr Corbyn’s aides say as many as 90 Labour MPs could back the government - and with 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.
Commons Speaker John Bercow said 157 MPs had applied to speak in the debate, but Labour and the SNP warned that not all would not get a chance and called on the government to clear two Commons days, something that was rejected by Commons leader Chris Grayling.
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.
Cameron faces apology calls
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. He faced calls to apologise for the comments at the start of the debate, from Labour, SNP and Lib Dem MPs. Mr Cameron said, “Everyone in this House should make up their mind on the arguments in this House and there’s honour in voting for, there’s honour in voting against. “That is the way this House should operate and that’s why I wanted to be absolutely clear at the start of my sentence that this is about how we fight terrorism.”