Commons votes against Syria action. A government motion was defeated by 285 to 272, a majority of 13 votes. Prime Minster David Cameron said it was clear Parliament does not want action and "the government will act accordingly".
It effectively rules out British involvement in any US-led strikes against the Assad regime.
Labour leader Ed Miliband said the vote meant military action was "off the agenda", and added that MPs had reacted against the prime minister's "cavalier and reckless" leadership.
The defeat comes as a potential blow to the authority of Mr Cameron, who had already watered down a government motion proposing military action, in response to Labour's demands for more evidence of Assad's guilt.
Labour had seen its own amendment - calling for "compelling" evidence - rejected by MPs by 114 votes.
But - in an unexpected turn of events - MPs also rejected the government's motion in support of military action in Syria if it was supported by evidence from United Nations weapons inspectors, who are investigating claims President Bashar al-Assad's regime had used chemical weapons against civilians.
Defence Secretary Philip Hammond confirmed that Britain would not be involved in any military action against the Assad regime - but said it would probably go ahead in any case.
"I expect that the US and other countries will continue to look at responses to the chemical attack.
"They will be disappointed that Britain will not be involved. I don't expect that the lack of British participation will stop any action."
He said he and the prime minister were "disappointed" with the result of the Commons vote which he said would harm Britain's relationship with Washington.
"It's certainly going to place some strain on the special relationship.
"The Americans understand the Parliamentary process. Perhaps they have been surprised by the scale of opposition."
Iraq 'poisoned the well'
Mr Hammond came under fire earlier for claiming Ed Miliband had given "succour" to the Assad regime by refusing to back Mr Cameron
Asked if all the MPs who voted against the government had given succour to President Assad, he said "The word was put in my mouth. The Assad regime is going to be a little bit less uncomfortable tonight as a result of this vote in parliament."
He blamed the 2003 Iraq war for "poisoning the well" of public opinion against British military interventions in the Middle East.
But Labour's shadow foreign secretary Douglas Alexander said the government defeat was down to the "fatally flawed" case put to MPs by Mr Cameron and Deputy Prime Minister Nick Clegg, claiming the pair's credibility was now "diminished".
But shadow defence secretary Jim Murphy said: "Parliament is entirely unpredictable sometimes and I don't think many people saw this coming."
He said so many of Mr Cameron's own MPs had voted with Labour because they were now "unwilling to take him at his word".
Conservative rebel Douglas Carswell said: "There is not now going to be British military involvement in Syria, but that is a good thing, the system works."
Leading Tory rebel John Baron said he was also "pleased the prime minister has listened" to Parliament. (bbc.co.uk)