Britain should join air strikes against the Islamic State (IS) group in Syria and shouldn't "sub-contract" its security to allies, British Prime Minister David Cameron says.
"I believe that we should now take the decision to extend British air strikes against ISIL into Syria," he wrote, ahead of a speech in parliament on Thursday (local time) where he will formally argue his case for war.
Cameron said it was in Britain's national security interests to strike IS jihadists and deny them a "safe haven" in Syria, arguing the burden should not fall only on Britain's allies.
"It is wrong for the United Kingdom to sub-contract its security to other countries," he said.
"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."
Cameron is expected to call a vote in parliament on the issue before recess begins on December 17.
He has stepped up pressure for MPs to back joining air strikes in Syria after IS claimed responsibility for the November 13 attacks in Paris, which killed 130 people.
Cameron on Monday visited Paris, where he met President Francois Hollande and paid tribute outside the Bataclan concert venue, where 90 people were killed.
"I firmly support the action President Hollande has taken to strike ISIL in Syria," Cameron said after talks in Paris.
"It's my firm conviction that Britain should do so too."
While British forces are taking part in air strikes on IS 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 IS.
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".