U.K. Prime Minister David Cameron suffered a big blow late Thursday, as Britain's House of Commons rejected his motion for military action in Syria as a response to a deadly chemical weapons attack by the Assad regime last week.

The motion was struck down by a 285-272 vote.

"It is clear to me that the British parliament, reflecting the will of the British people, does not want military action," Cameron said after the vote.

He added that he would "act accordingly."

Secretary of Defence Philip Hammond told BBC that there will be no British military intervention in Syria.

The U.S. took notice of Britain's waning enthusiasm for intervention. Hours before the vote, White House aides leaked that President Barack Obama would be willing to engage alone in Syria. France said Thursday that its military is preparing for possible action, and reiterated that willingness on Friday morning.

The vote in Britain came after hours of heated debate in Parliament, and Cameron's comments signal that the U.S. has potentially lost a key ally in any military involvement in Syria.

It was a humiliating blow for Cameron, as Paul Goodman, Editor of Conservative Home, noted on Twitter:

"A breach with America finally comes when a Conservative Prime Minister is in Downing Street - and one on good terms with President. This vote is the biggest foreign policy reversal inflicted on a Prime Minister by the Commons in my lifetime."

The White House shrugged off any implication it would have on Obama's decision-making.

"We have seen the result of the Parliament vote in the UK tonight," White House spokeswoman Caitlin Hayden said in an email.

"The U.S. will continue to consult with the U.K. Government – one of our closest Allies and friends. As we’ve said, President Obama’s decision-making will be guided by what is in the best interests of the United States. He believes that there are core interests at stake for the United States and that countries who violate international norms regarding chemical weapons need to be held accountable."

The New York Times reported that the U.S. could strike Syria as soon as Saturday, after United Nations inspectors investigating the chemical attack leave the country. Senior administration officials are expected to lay out their case to members of Congress on intervention in Syria in a 6 p.m. ET conference call Thursday evening.

In addition to facing a defection from a key ally, Obama has seen more members of Congress question his plans on Syria. House Speaker John Boehner spoke with Obama on Wednesday, a day after he sent Obama a letter asking him for answers on 14 questions relating to possible military action in Syria. Boehner came away from that meeting with some questions still unanswered.

"It is clear that further dialogue and consultation with Congress, as well as communication with the American public, will be needed," said Brendan Buck, Boehner's spokesman.

Earlier on Thursday, the British government had laid out its own legal rationale for intervention in Syria, arguing that it was justified on humanitarian grounds.

The rationale outlined the use of chemical weapons "by the Syrian regime" of President Bashar al-Assad as a "serious crime of international concern." It argues that the aim of any military response would be to "relieve humanitarian suffering by deterring or disrupting the further use of chemical weapons."

