Jeremy Corbyn: "Does it strengthen or undermine our security?"
Labour leader Jeremy Corbyn has written to his MPs saying he cannot back UK air strikes in Syria - prompting a warning of shadow cabinet resignations.
Mr Corbyn rejected David Cameron's claim that targeting so-called Islamic State would make Britain safer.
His intervention - which puts him at odds with a number of his MPs - was criticised by a shadow cabinet member.
The frontbencher said there would be resignations if Mr Corbyn ordered the shadow cabinet to back his stance.
Labour is divided on whether to support Mr Cameron's call for air strikes, with about half of the shadow cabinet believed to back intervention.
Mr Corbyn's decision to write to MPs before the shadow cabinet had reached agreement was criticised by a senior shadow minister, who told BBC assistant political editor Norman Smith: "There will be resignations among senior members of the shadow cabinet over this."
He said Mr Corbyn's letter had led to a breakdown of trust within the shadow cabinet, which had not been consulted before it was sent.
A British serviceman would be "insulted" by Mr Corbyn's "ridiculous party games" he said, adding: "We are becoming a complete joke."
Shadow foreign secretary Hilary Benn had earlier told BBC political editor Laura Kuenssberg he found the case for strikes "compelling".
David Cameron on IS: "They have already tried to attack us"
Hilary Benn tells Laura Kuenssberg 'compelling case' for Syria action
Downing Street sources said: "It's not a great surprise that Jeremy Corbyn has come out against air strikes. The issue is how many Labour MPs feel able to support him."
Shadow international development secretary Diane Abbott told Channel 4 News Thursday's shadow cabinet meeting had been "lively".
She said 70% of Labour members were against air strikes, but that some MPs "seem to have been persuaded by David Cameron".
After "a weekend's reflection", she predicted, these MPs would "see things differently".
The Labour row comes after Mr Cameron spent nearly three hours trying to convince MPs action against IS in Syria would make Britain "safer" and would be part of a "comprehensive" strategy alongside key allies to defeat the group.
He told MPs the UK was already a target for IS and could not "outsource our security to allies".
The prime minister said he would only hold a Commons vote authorising action if he was certain he could win it - and that will depend on persuading Labour MPs to back him.
The vote could come as soon as next week, with former Tory rebels and some Labour backbenchers indicating they now backed action.
Labour's shadow cabinet met after Mr Cameron's statement and Mr Corbyn said it would continue trying "to reach a common view" on Syria on Monday.
But in his letter to MPs, he said: "We've all been horrified by the despicable attacks in Paris and are determined to see the defeat of ISIS.
"Our first priority must be the security of Britain and the safety of the British people. The issue now is whether what the PM is proposing strengthens, or undermines, our national security.
"I do not believe that the PM today made a convincing case that extending UK bombing to Syria would meet that crucial test. Nor did it satisfactorily answer the questions raised by us and the Foreign Affairs Committee.
"In particular, the PM did not set out a coherent strategy, coordinated through the UN for the defeat of ISIS. Nor has he been able to explain what credible and acceptable ground forces could retake and hold territory freed from ISIS control by an intensified air campaign.
"In my view, the PM has been unable to explain the contribution of additional UK bombing to a comprehensive negotiated political settlement of the Syrian civil war, or its likely impact on the threat of terrorist attacks in the UK.
"For these, and other reasons, I do not believe the PM's current proposal for air strikes in Syria will protect our security and therefore cannot support it."
Foreign Secretary Philip Hammond earlier said he believed the government was building a consensus for military action.
He told BBC News: "From what I have seen from the response to the prime minister's statement today it feels to me that we are building a consensus now for military action in Syria as part of a package of British response which will include a humanitarian and a very strong diplomatic/political strand as well."