Jeremy Corbyn has granted Labour MPs a free vote on UK air strikes in Syria.
Mr Corbyn had wanted Labour to oppose air strikes, but was forced to back down by his shadow cabinet.
David Cameron later said there would be a Commons debate and vote on Wednesday, saying he believed there was growing parliamentary support for action.
BBC chief political correspondent Vicki Young said she was told Mr Corbyn was given a "thorough kicking" at the meeting with his shadow cabinet.
He had previously suggested he wanted to agree a united position within his shadow cabinet and for Labour to approach the question "as a party". The free vote means Labour MPs will not be ordered to vote with the leadership.
The prime minister had said he would only call a vote when he could be confident of a win. He needs enough Labour MPs to back military intervention to make up for Conservative MPs who oppose the action.
Labour had called for a two-day debate ahead of any vote, with Mr Corbyn saying a single day "would inevitably lead to important contributions being curtailed".
But Mr Cameron said the government would "take the action necessary" to ensure the debate lasted the equivalent number of hours of a debate taken over two days.
The UK parliament's third largest party - the SNP - opposes bombing IS in Syria, with leader Nicola Sturgeon criticising Labour's stance, tweeting that "a party that says it is anti-air strikes has just made a vote for air strikes more likely".
A number of Labour frontbenchers are believed to support Mr Cameron's calls for air strikes and the Labour leader was warned of resignations if he attempted to force his party to vote with him.
But ahead of the shadow cabinet meeting, the Labour leadership said 75% of party members it polled over the weekend had opposed bombing.
The survey was called into question with one Labour MP calling it a "rather vague consultation".
Labour MP, and former shadow international development secretary, Mary Creagh said she thought Mr Corbyn had made the right decision.
"I'm glad that we have come down on the side of a free vote," she said. "It was clear that any vote to support our French and US allies in Syria attacking the murderous barbarism of Islamic State was going to cause difficulties for the party and has split the party right down the middle, so I think we're in the right place tonight."
But John Woodcock MP, who is in favour of airstrikes, questioned whether the vote would be genuinely free, with "certain people floating around the edges and warning of the dire consequences to their political future if they vote in the way that they believe to be right, which just happens to be in disagreement with the leadership".