LONDON — A sharply divided British Parliament on Thursday rejected the use of military force as a response to suspected chemical attacks in Syria, depriving Washington of assistance from its most trusted ally if the U.S. launches a military strike against the Syrian government.
Hours of impassioned debate in the House of Commons culminated in a 285-272 vote against a motion to condemn the alleged use of poison gas against Syrian rebel strongholds and to uphold military reprisal as a legitimate option against the government of President Bashar Assad.
Defense Secretary Philip Hammond later confirmed to the BBC that Britain would sit out any military action against Syria. He expressed concern that Britain's "special relationship" with the U.S. would come under strain.
It was a humiliating and unexpected defeat for Prime Minister David Cameron, one of the most vocal advocates of an aggressive response to Assad's alleged use of chemical weapons. Although he technically does not need Parliament's approval to launch military action, Cameron said he would abide by the will of lawmakers, many of whom cited the disastrous war in Iraq as a reason to refrain from intervening in Syria.
"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," a tense-looking Cameron said immediately after the vote.
Lawmakers, including some of Cameron's fellow Conservatives, demanded restraint until United Nations weapons inspectors issue their findings from an investigation in Syria. That demand was similar to calls from France and other European nations.
The hesitation on this side of the Atlantic means that, should President Obama decide to launch a strike against Syria within the next few days, he will probably have to do so without the backing — or help — of some of Washington's best friends.
The leader of the opposition Labor Party said Britain should make its own decisions in its own time, without regard to external pressure.
"There mustn't be a rush to judgment" because of "an artificial timetable set elsewhere," Labor leader Ed Miliband said.
His party voted en bloc Thursday against Cameron's motion for an endorsement, in principle, of a possible military intervention in Syria. The majority against the motion also included votes from dissenting lawmakers in the two parties that form the coalition government: the Conservatives and the Liberal Democrats.
Cameron already had been forced to water down the motion he was intending to introduce in the House of Commons asking for immediate authorization of a strike on Damascus. He had summoned members of Parliament back early from their summer recess to vote on the motion.
Political and public opposition in Britain to a strike against Syria has been growing in recent days, fueled by bitter memories of the decision to join the U.S. in its 2003 invasion of Iraq based on false claims of the existence of weapons of mass destruction there. Several lawmakers in the House of Commons referred to the Iraq war as an object lesson to not rush into action in the volatile Middle East without due regard to potential consequences.
The prime minister acknowledged those worries, which he passed on to Obama in a recent phone conversation.
"I also explained to him that, because of the damage done to public confidence by Iraq, we would have to follow a series of incremental steps, including at the United Nations, to build public confidence and ensure the maximum possible legitimacy for any action," Cameron told Parliament.
His government put forward a resolution in the U.N. Security Council on Wednesday authorizing military retaliation against Assad's forces, but the measure has little chance of passing, given opposition by China and Russia. Cameron now cannot even claim the backing of his Parliament.
French President Francois Hollande, who also has been at the forefront of the campaign for a forceful response to Assad, seemed to temper his words somewhat, saying Thursday that "everything must be done to reach a political solution."
Hollande insisted that the use of chemical weapons must not go unpunished; in an interview published Monday, he promised that Western powers would decide "this week" what to do. But he has shied away from explicitly backing military intervention.
Germany, too, has been vague about what consequences the Syrian government should face. And the Italian government has declared that it would not participate in any operation on Syria without U.N. sanction.
To counter skeptics, Cameron's office took the unusual step Thursday of making public an assessment of the situation in Syria by its Joint Intelligence Committee and the government's legal justification for military intervention.
The committee described it as "highly likely" that Assad's forces were behind a suspected major chemical attack last week that reportedly killed hundreds, many of them civilians. It said that rebels were almost certainly not capable of mounting such an attack and that evidence from many sources supported its conclusion as to the Assad government's culpability.
The committee expressed puzzlement as to "the regime's precise motivation for carrying out an attack of this scale at this time." Still, Cameron said, allowing the attack to go unanswered would only encourage Assad to do it again.
As for the legality of a military reprisal, British officials said it was permissible "as an exceptional measure on grounds of overwhelming humanitarian necessity." Any operation would be limited in duration and restricted to the sole objective of striking specific targets to deter Assad from launching chemical attacks and to alleviate human suffering.
But John McDonnell, a Labor Party lawmaker, said that alternatives to force remained, including pushing harder for a diplomatic settlement between the warring sides. The intelligence committee's cautious language was also too flimsy a basis on which to unleash the British military, he said.
"'Highly likely and 'some evidence' are not good enough to risk further lives, risk counterattack, inflame the whole region, risk dragging other states into this war and, at the same time, increase the risk of terrorism on British streets," McDonnell said.
Paddy Ashdown, a former leader of the Liberal Democrats, the junior partner in the coalition government, said in the House of Lords that ultimately the decision to take military action was a judgment call.
"You can't tell exactly what's going to happen. You can't have certainty," said Ashdown, who nonetheless supported Cameron's motion.
Polls in Britain show that a majority of people oppose a military strike. Antiwar protesters are expected to stage a demonstration in London on Saturday.