Jeremy Corbyn has written to Labour MPs to tell them he cannot personally support airstrikes against ISIS in Syria.
The Labour leader, a former chair of the Stop the War Coalition, said David Cameron had failed to "set out a coherent strategy" when he made the case for extending UK strikes into Syria in the Commons earlier on Thursday. A vote on military action on Syria is widely expected to happen next week.
Corbyn's letter was released shortly after a 90-minute meeting of the shadow cabinet at which a majority of members were in support of military action.
Corbyn wrote that the shadow cabinet was yet to agree a "common view" but did not comment on whether Labour MPs would be allowed a free vote on the matter.
In the prime minister's statement to the Commons, Cameron said it was wrong to "shirk our responsibility" and leave the US, France, and other allies to fight the terrorists in the wake of the Paris terror attacks. He added: "If not now, when?"
But Corbyn, a lifelong anti-war campaigner, wrote: "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 today made a convincing case that extending UK bombing to Syria would meet that crucial test."
He added: "For these and other reasons, I do not believe the prime minister's current proposal for air strikes in Syria will protect our security and therefore cannot support it."
He said the shadow cabinet would try to agree a "common view" on Monday.
But Corbyn's decision to publicly reveal his position at this stage leaves the party in an uncomfortable position, with senior frontbenchers and many MPs in favour of action but the leader against.
Earlier on Thursday, shadow foreign secretary Hilary Benn told BBC News the arguments in favour of action were "compelling" and said there was a very strong case for Britain playing "our full part".
Here is Corbyn's letter in full.
Emily Ashton is a senior political correspondent for BuzzFeed News and is based in London.
Contact Emily Ashton at email@example.com.
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