British lawmakers voted against military action in Syria on Thursday in a stunning defeat for Prime Minister David Cameron.

The government was defeated by just 13 votes in the House of Commons in its bid for a "strong humanitarian response" to the alleged use of chemical weapons by the Syrian regime.

Minutes after the shock result, Cameron told lawmakers: "It is clear to me that the British parliament, reflecting the views of the British people, does not want to see British military action.

"I get that, and the government will act accordingly."

Cameron's defeat raises the prospects that the United States could act alone against the government of Syrian President Bashar al-Assad, which it blames for horrific poison gas attacks that are believed to have killed hundreds in the suburbs of Damascus last week.

"Britain will not be involved in any military action," a spokesman for Cameron's Downing Street office confirmed.

Seven hours of impassioned debate in the House of Commons had revealed deep divisions over whether military strikes against the Assad regime would deter the further use of chemical weapons, or simply make the conflict worse.

Cameron had made the case for targeted strikes, insisting that Britain could not stand idle in the face of "one of the most abhorrent uses of chemical weapons in a century".

"If nothing is done, (the regime) will conclude that it can use these weapons again and again, and on a larger scale, and with impunity," he had told parliament.

But he faced strong resistance from the opposition Labour party and many members of his own Conservative party, who expressed fear that Britain was rushing to a war without conclusive evidence that Assad had gassed his own people.

Cameron admitted there could not be "100 percent certainty" about who had committed the attack, but said it was "beyond doubt" that the regime was responsible.

Britain's Joint Intelligence Committee released evidence Thursday saying that chemical weapons had undoubtedly been used on August 21 and that more than 300 people had been killed.

It was "highly likely" that the Syrian government was responsible, it added.

The regime has denied it committed the attack, which it blames on rebel forces.

Britain's Labour party had proposed an alternative motion seeking "compelling" evidence that the regime carried out the attack before committing to any form of military action, but this too was defeated.

Cameron had recalled MPs from their summer break for the emergency debate, which saw the government's motion defeated by 285 votes to 272.

In a severe blow to the prime minister's authority, 30 of his own Conservatives voted against the motion, along with nine members of his junior coalition partners, the Liberal Democrats.

Defence Secretary Philip Hammond warned that the result would "put a strain" on Britain's so-called special relationship with the United States.

"The Americans do understand the parliamentary process that we have to go through," he told BBC television.

"They have always understood that in order to be involved in military action we would have to secure the consent of parliament."

The spectre of the Iraq war came up many times during the long hours of debate.

In 2003, the British parliament gave then prime minister Tony Blair the go-ahead to join the US-led invasion of Iraq on the basis of allegations that dictator Saddam Hussein had weapons of mass destruction.

The weapons never materialised and Britain became embroiled in the war for years.

But Cameron insisted: "This is not like Iraq. What we are seeing in Syria is fundamentally different."

In a humiliating climbdown, Cameron had already been forced to agree late Wednesday that Britain would not take part in any military strikes before United Nations inspectors report back from the site of the attack -- meaning a second vote, possibly next week, would have been necessary.

Opinion polls suggest that the British public are strongly against involvement in military strikes against Syria.

A YouGov poll for The Times found that only 22 percent supported firing missiles against Syrian military sites.

Britain had dispatched six Typhoon fighter jets to its Akrotiri base on Cyprus as a "protective measure" on Thursday, although the defence ministry said the planes would not take part in any direct military action.