British Prime Minister David Cameron has lost his parliamentary vote calling for military action over Syria "in principle".
Cameron said it was clear the British Parliament does not want action and "I will act accordingly".
The government motion was defeated 285 to 272, a majority of 13 votes.
It effectively rules out British involvement in any US-led strikes against the Assad regime.
And it comes as blow to the authority of David Cameron, who had already watered down a government motion proposing military action, in response to the opposition Labour Party\’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 says the US "will be disappointed that Britain will not be involved" in any military strike. "I don\’t expect that the lack of British participation will stop any action," he adds.
The permanent members of the United Nations Security Council made no comment after meeting to discuss a British resolution calling for international military action in Syria after an alleged chemical weapons attack.
Envoys from the United States, Britain, France, Russia and China gathered on Thursday afternoon at UN headquarters in New York. The meeting was called by the Russian delegation, and ended with no statement from the participants.
A similar meeting on Wednesday ended after more than an hour with no agreement.
Earlier on Thursday, Syrian President Bashar al-Assad said that his country would defend itself against any foreign military intervention.
"Syria will defend itself in the face of any aggression, and threats will only increase its commitment to its principles and its independence," the embattled Syrian leader told a visiting delegation of Yemeni politicians, according to state media.
Washington and its European allies have made clear they think the government of Syrian President Bashar al-Assad is responsible for the attack, and have said a military response is needed.
The U.S. on Thursday sent a fifth navy destroyer to the Mediterranean; Britain is reinforcing its military on Cyprus, and Israel is bolstering its forces along the Syrian border.
Even as military assets take position, the U.S. and its allies face growing opposition, both at home and abroad, to a strike against Syria.
In the U.S., more than 100 members of Congress, including several from President Barack Obama’s Democratic Party, signed a letter demanding that the administration consult with Congress before any military action.
U.N. Secretary General Ban Ki-moon has called for patience and insists he will receive a report on Saturday from weapons inspectors investigating the attack.
Ban said the team would leave the country by Saturday and report its findings to him. He said "diplomacy should be given a chance" before action was taken.
The U.N. investigators carried out their third inspection Thursday of the alleged chemical weapons attack last week. Hundreds of civilians are thought to have died in the incident.
China and Russia, both permanent Security Council members, have blocked previous attempts to impose sanctions on President Assad’s regime. That has led to frustration for the U.S. and its European allies.
The Syrian government denied having any role in the alleged gas attacks. And state media quoted President Assad Thursday as saying his country would defend itself against any aggression and emerge "victorious."
On Syrian state TV, U.N. Ambassador Bashar Jaafari said the U.S. and its Western allies were using the threat of a military strike to prevent the inspectors from carrying out their work and to influence their conclusions.
Al Jaafari said the Syrian government would like to see the team continue its work freely and objectively, without any political or military pressure.
Iranian President Hassan Rouhani has said his government and Russia will work to prevent an attack on Syria, and warned any assault could “bring great costs” to the region.
Iran also has warned any Western action against Syria would result in the "imminent destruction of Israel," a U.S. ally in the region.
And also Thursday, Canadian Prime Minister Stephen Harper said his government supports Washington and its other allies contemplating a military response to Syria. However, he said, the Canadian military would not take a role in any attack.
Later on Thursday, President Barack Obama\’s top national security advisers plan to brief members of Congress about intelligence on the gas attacks.
Obama said he has not decided on any action, but he has vowed that those who break international norms need to be held accountable.
The Speaker of the Syrian parliament has written to his counterpart in London inviting a British parliamentary delegation to visit Damascus as soon as possible.
French President Francois Hollande has also yet to decide about a military intervention. But on Thursday, after meeting Ahmed Jarba, the head of the opposition Syrian National Coalition, Hollande said a political solution would only be possible if "the international community can put a temporary stop to this escalation in violence".
Syria has been embroiled in a war for more than two years, during which more than 100,000 people have been killed and millions have been displaced or become refugees in other countries, according to the United Nations.