Jeremy Corbyn has emailed Labour party members in Britain to ask whether they think UK's parliament should vote to authorise airstrikes against Islamic State in Syria.
The leader's consultation on what the UK should do comes amid speculation that politicians could vote on the issue next week.
In the message sent on Friday night, Mr Corbyn wrote: "We have all been horrified by the despicable attacks in Paris and are determined to see ISIS defeated.
"The issue now is whether what the Prime Minister is proposing strengthens, or undermines, our national security.
"I do not believe that the Prime Minister made a convincing case that British airstrikes on Syria would strengthen our national security or reduce the threat from ISIS."
Mr Corbyn, who wants members to give their thoughts on the airstrikes by the start of next week, is facing a shadow cabinet revolt over the issue.
Many of his colleagues have expressed shock at his opposition to bombing IS targets, while some are accusing him of a "failure of leadership".
It is reported that British government officials have been approaching Labour MPs for briefings over the weekend to try to shore up support.
Earlier on Friday, French president Francois Hollande urged all British MPs to back David Cameron's plans to expand strikes against the terror group from Iraq to Syria.
On Thursday, the David Cameron pressed his "compelling" case, saying the strikes would make Britain safer.
However, Mr Cameron has said he will not go ahead with a vote in the House of Commons until he is certain of victory.
The British government was humiliated after a previous vote on strikes in Syria, which were intended to deter the use of chemical weapons by the Assad regime, was rejected in 2013.
France has stepped up its own strikes in Syria after IS terrorists killed 130 people in Paris two weeks ago - an atrocity Mr Hollande called an "act of war".
Mr Hollande has met the leaders of Russia, the US, Germany and Britain this week as he tries to form an international alliance to crush Islamic State.
Meanwhile, a poll shows that the British public backs airstrikes in Syria by a margin of 48% to 30%.
But the research by Survation for the Daily Mirror found the remainder are undecided - and 59% believe the move would increase the risk of terrorist attacks in the UK.
