The Government lost the Commons vote on the motion to support military intervention in Syria after a fraught parliamentary debate on Thursday. The motion was defeated by 285 to 272 - a majority of 13.
Following the announcement, David Cameron said it was clear Parliament "does not want to see British military action" in Syria after the Government was defeated on the issue, adding: "I get that, and the Government will act accordingly."
A Labour amendment demanding "compelling evidence" that the Syrian regime was responsible for a chemical attack was won by the Government.
During a rancorous debate, Ed Miliband was accused of giving “succour” to the Assad regime following his refusal to back the government’s push on authorising British military intervention.
The Syrian government is accused of launching an attacked on its own people last week, leading to hundreds of deaths; however UN weapons inspectors have yet to publish their report, despite claims from Western governments that there is clear evidence that the regime was responsible.
The revelation that Miliband was to oppose the Commons vote, a story broken exclusively by the HuffPost UK on Wednesday evening, meant that a second vote was needed.
Anger towards Miliband from the government ranks was palpable as the debate descended into heated exchanges between the tow main parties.
Earlier, Labour representatives had sent a letter to Cabinet Secretary Sir Jeremy Heywood complaining of the "infantile and irresponsible" comments made by the Tory Director of Communications who had also accused the Labour leader of “giving succour” to the Assad regime.
The Defence Secretary Philip Hammond had repeated the remarks on Thursday evening, adding: "Anything that stops us from giving a clear united view of the British Parliament tonight will give some succour to the regime."
"We deliberately structured our motion to take account of the concerns the Leader of the Opposition had expressed directly to us. But he has still chosen to table an amendment and ensure that we don't have a clear, united and unified opinion from the British Parliament."
The Government lost the vote on Syria after a fraught political debate
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Miliband's insistence that a decision on UK involvement should await the report of UN weapons inspectors examining the attack site in Damascus forced Mr Cameron to water down his motion.
Without Labour's support, the vocal scepticism of many Conservative MPs meant the Prime Minister was faced with a humiliating Commons defeat, unprecedented in the 20th century.
He accepted the need for a second vote to authorise any involvement.
But the concessions did not go far enough for Labour leader Ed Miliband, who tabled an alternative motion demanding "compelling evidence" that the Assad regime was responsible.
Making the case for military action as he opened the debate, Cameron conceded that there could be no "100% certainty" about who committed the chemical weapons attack in Syria.
The Prime Minister said there was not "one smoking piece of intelligence" but insisted he was convinced by the evidence that it was "beyond doubt" Bashar Assad's regime was responsible.
Documents published by Downing Street showed Britain would be permitted to launch a targeted strike on humanitarian grounds, even if Russia and China block an agreement at the United Nations.
Evidence from the Joint Intelligence Committee found that a chemical weapons attack did occur in Damascus last week and it is "highly likely" that Bashar Assad's regime was responsible.
The Syrian leader said the country would "defend itself" against any aggression.
Miliband did not rule out backing military action in a subsequent Commons vote after a chance to see and assess evidence "in a calm and measured way".
But the party's decision would depend "on the case that has been set out and the extent to which international support has been developed", he told MPs.
The Prime Minister "has to make a better case than he did today", he added.
Labour sources condemned the "personal insults" from Government sources in the run-up to this afternoon's debate.
According to The Times, a Government source said: "No 10 and the Foreign Office think Miliband is a f****** c*** and a copper-bottomed s***."
"Downing Street should not lower itself to the level of personal abuse," the Labour source said, blaming the Government for any lack of consensus.
Thursday's vote is a test of Cameron's authority - but the debate has exposed the depth of concern among MPs of all parties which will make it very hard for him to win later approval for direct action.
The spectre of the decision by Tony Blair to take Britain into war with Iraq a decade ago on the basis of intelligence about weapons of mass destruction that later proved false hung over the debate.
Hammond said he recognised that public and parliamentary opinion had been "poisoned by the experience of the Iraq dodgy dossier".
It meant the Government would have to "go the extra mile" to convince people, he said, after opinion polls showed public support for military intervention falling to just 22%.
The United States is expected later to publish its own dossier of evidence that the Assad regime was responsible for the chemical weapon attack which killed hundreds of civilians last week.
And the permanent members of the UN Security Council - the UK, America, France, Russia and China - are to meet this evening to discuss the situation.
The UK has tabled a draft resolution seeking approval for military action.
But Moscow, a key ally of Assad, is opposed to any military intervention and with China has vetoed all previous attempts to secure resolutions critical of the regime and imposing sanctions.
"Nobody would be happier than us if we could get a UN resolution. The reality is we would expect the Russians to veto it," Mr Hammond said.
The team of weapons inspectors is expected to complete its work tomorrow and leave the Syrian capital the following morning and deliver its initial findings to Secretary-General Ban Ki-moon.
Mr Ban has urged the international community to "give peace a chance" by awaiting the results - although the team's remit extends only to whether banned weapons were used not who used them.
Under the terms of the Government motion, the Security Council would need to have considered the initial findings and "every effort" made to get agreement to military action before MPs could be asked again to back British involvement in air strikes.
Mr Cameron said the biggest danger of escalation in the Syrian civil war, which has so far cost more than 100,000 lives, was for the world to "stand back and do nothing".
"I think Assad will draw very clear conclusions from that," he said - warning that the regime could increase its use of chemical weapons.
The Syrian leader said the country would "defend itself against any aggression".
Former Tory leadership contender David Davis told Channel 4 News there was no reason to withhold evidence.
"Normally with intelligence you have got to be very careful, but... the Americans are releasing intercepts - that's normally the Holy Grail. If they can release that… they can release anything.
"The fact of the matter is, as we stand here today, we have not had a case for intervention. We do not know whether it was Assad who instructed the use of the nerve gas.
"We don't know whether it was a rogue element in his forces that did it, we don't even know if it was the rebels that did it.
There's lots more to discover. Until we do that we should not be attacking."
He dismissed suggestions that a delay would give "succour" to Assad: "No, it's not - you always find governments want to rush into things."
Former Cabinet minister Andrew Mitchell told the programme: "I think it's very important that all the evidence is published that can be."
He added: "A very sceptical British public won't accept politicians saying they have seen the evidence and that's enough. They need to hear themselves what the evidence is that would lead to military action by Britain. That's because of the legacy of the Iraq War and that's why it's so important this time."