Prime Minister David Cameron on Thursday argued his case to MPs for Britain to join air strikes in Syria ahead of a vote expected at a later date, with signs of opposition weakening after the Paris attacks.
"If we won't act now, when our friend and ally France has been struck in this way, then our friends and allies can be forgiven for asking: If not now, when?" Cameron asked parliament.
Cameron argued there was a legal basis for intervention for self-defence because of the threat posed by ISIS at home, and said Britain should not "sub-contract" its security to allies.
"We have to deny a safe haven for ISIL in Syria. The longer ISIL is allowed to grow in Syria, the greater the threat it will pose," he said in a written statement on the issue, using another word for ISIS.
Cameron called for "patience and persistence" and outlined a seven-point strategy for Syria, including diplomatic and humanitarian efforts and planning for what will happen if President Bashar al-Assad falls.
Cameron is expected to call a vote in parliament on the issue before recess begins on December 17.
This will come two years after a previous vote for military action in Syria failed after the main opposition Labour Party voted against.
Cameron has stepped up pressure on MPs to vote for strikes after ISIS claimed responsibility for the November 13 attacks in Paris, which killed 130 people.
"The events in Paris have clearly changed things," Malcolm Chalmers, research director at the Royal United Services Institute (RUSI), told AFP.
"I think the mood in parliament has changed," he said, predicting that the vote will pass since "a significant number of MPs" had changed their minds.
"There's scepticism on both sides of the Houses but I think opinions are beginning to change," he said.
But Chalmers also said there was still a "shadow" from Britain's participation in the US-led invasion of Iraq in 2003 and the NATO bombing campaign in Libya in 2011 which helped topple dictator Moamer Kadhafi but was followed by bitter civil war.
Critics have argued that joining the campaign could increase the risk of Britain becoming a target.
"As long as we intervene in the Middle East, we must expect atrocities in return. Bombing will not stop them," columnist Simon Jenkins wrote in the Evening Standard this week.
But Cameron on Thursday argued that Britain was already a target, pointing to the killings of 30 British tourists by an ISIS gunman in a Tunisian resort in June in which a total of 38 people were killed.
He also said that Britain was already assisting in the air campaign on Syria with surveillance.
While British forces are taking part in air strikes on ISIS targets in Iraq, they are not involved in the US-led coalition targeting Syria due to resistance from opposition parties still mindful of previous unpopular interventions in Iraq and Afghanistan.
Labour's anti-war leader Jeremy Corbyn is against any military action but Cameron appears increasingly confident he can get enough support from Labour MPs to pass the vote, particularly after last week's UN Security Council resolution authorising countries to "take all necessary measures" against ISIS.
A Times/YouGov opinion poll last week found that 58 percent of people would approve of Britain joining air strikes in Syria, compared to 22 percent against.
Reports suggest the government could call a vote on the issue next week.
Cameron on Monday only said that the vote could come "in the coming days and weeks".