LONDON — Invoking the specter of the Iraq war, British lawmakers on Thursday rejected military action in Syria, dealing a stunning blow to Prime Minister David Cameron and effectively ruling Washington’s staunchest military ally out of any U.S.-led strike.

After a marathon eight-hour debate, Cameron lost a vote that was initially seen as a symbolic motion setting up a final vote in the days ahead authorizing force against Syrian President Bashar al-Assad’s regime for allegedly using chemical weapons. But the surprise loss of even the weaker piece of legislation — by a vote of 285 -272, including a group of rebels from Cameron’s Conservative Party in opposition — appeared to cost the United States its centerpiece ally in a still-forming coalition. The rejection additionally signaled what analysts called the biggest rupture in the U.S.-British “special relationship” since the 1982 Falklands war.

Technically, Cameron could still authorize military strikes over the objection of Parliament, but top government officials — including the prime minister himself — indicated that was not an option following Thursday’s defeat.

“It is clear to me that the British Parliament, reflecting the views of the British people, does not want to see British military action,” Cameron said after losing the vote. “I get that, and the government will act accordingly.”

The rejection — which dealt Cameron the most powerful setback of his premiership and came as one lawmaker shouted “resign” as the vote was read out — amounted to an extraordinary turn of events in Britain. Only two days ago, this nation appeared ready to fast-track a plan to join a U.S.-led coalition.

But over the past 24 hours, Cameron, a hawk on Syria who has long argued for a tougher response, has encountered a level of domestic political resistance that caught his government off guard. It suggested the extent of the damage done here from the faulty intelligence and mission creep that steered British troops into Iraq a decade ago.

During the debate in the House of Commons, Cameron confronted an avalanche of skepticism. In his impassioned call for action, the prime minister defended President Barack Obama and U.S. humanitarian motives in Syria, while also acknowledging that Britons — who polls show are overwhelmingly opposed to military intervention — were understandably gun-shy after the mishaps in the run-up to the 2003 invasion of Iraq. “The well of public opinion has been well and truly poisoned by the Iraq episode,” he said.

The door, observers said, could be open to indirect military cooperation, including intelligence-sharing. But any direct military involvement — such as British missiles being launched into Syria — now appeared largely out of question.

The reluctance to back a possible U.S.-led mission, analysts said, marked a rare and major jolt to U.S.-British relations, the strategic pillar of transatlantic policy for decades that has seen Washington and London forge one of the closest military alliances of modern times.

“To not support the U.S. would be very damaging to the U.K., damaging to our relationship with the U.S. and to our global standing,” said Richard Kemp, the former commander of British forces in Afghanistan. “Britain would be diminished if we didn’t fight.”

Cameron repeatedly sought to reassure skeptical lawmakers that he was not contemplating another Iraq. “It is not about taking sides in the Syrian conflict, it is not about invading, it is not about regime change, and it is not even about working more closely with the opposition,” Cameron said. “It is about the large-scale use of chemical weapons and our response to a war crime — nothing else.”

His government sought to back up the argument for action by releasing a brief intelligence assessment of the Aug. 21 incident near Damascus, as well as a document offering legal backing for action. Bowing to earlier demands from the opposition Labor Party, he had pledged that Britain would take no action until the United Nations had reviewed a report from weapons inspectors, who are in Syria examining the sites of an alleged chemical attack last week that left hundreds dead.

While Cameron conceded that there was no “smoking piece of intelligence,” he argued that reasonable assessments indicated that the Assad government was responsible for the deadly attack. The West, he said, had an obligation to deter a possible repeat of “one of the most abhorrent uses of chemical weapons in a century” — an apparent reference to a 1988 incident in Iraq when Saddam Hussein ordered a poison gas attack in the Kurdish town of Halabja that killed thousands of people.

Yet skeptical lawmakers repeatedly cautioned that the strikes could end up aiding al-Qaida-backed elements within the Syrian opposition and could escalate violence while failing to deter a repeat of chemical weapons use. Many labeled the government’s intelligence inadequate and insisted that despite Cameron’s promises, Britain could be dragged into a protracted military operation.

“Those of us who were here in 2003, at the time of the Iraq war, felt they had their fingers burnt,” said Richard Ottaway, a conservative member of Parliament.

Other lawmakers, meanwhile, had sought to dispel the ghost of Iraq. “This is not Iraq, we are not putting boots on the ground and we are not invading,” longtime lawmaker Paddy Ashdown argued during a separate debate in the House of Lords on Thursday. “And above all, this is not George W. Bush. It’s Obama.”

Cameron was particularly at loggerheads with the opposition Labor Party leader, Edward Miliband, who had deepened his resistance to military strikes during the past 24 hours. In an oddly halting performance in Parliament on Thursday, he did not rule out supporting military action, but he called for “a calm and measured” analysis. “I do not believe we should be rushed to judgment on this issue based on a political timetable set elsewhere,” he said in a clear reference to Washington.

The government first sensed major trouble after several hours of debate, following an announcement by Miliband that Labor lawmakers would vote against Syria action — marking the first time in nearly five decades that a British opposition party has rejected a government motion for military intervention.

The true depth of his opposition, however, had remained unclear, with some analysts predicting he was merely staking out a political position against a preliminary motion for action that would be reversed after U.N. weapons inspectors produced a report in the coming days. Yet the additional defection of lawmakers from the ruling coalition of Conservatives and Liberal Democrats away from the government motion ultimately doomed Cameron.

On Thursday, Miliband, however, had made one thing clear from the beginning. “We have got to learn the lessons of Iraq,” he said in Parliament.

Karla Adam contributed to this report.