By Paul Lewis and Spencer Ackerman, Guardian UK Obama's Air Strike Plans in Disarray!

Barack Obama's plans for air strikes against Syria were thrown into disarray on Thursday night after the British parliament unexpectedly rejected a motion designed to pave the way to authorizing the UK's participation in military action. The White House was forced to consider the unpalatable option of taking unilateral action against the regime of Bashar al-Assad after the British prime minister, David Cameron, said UK would not now take part a in any military action in response to a chemical attack in the suburbs of Damascus last week. Although Britain's support was not a prerequisite for US action, the Obama administration was left exposed without the backing of its most loyal ally, which has taken part in every major US military offensive in recent years. Caitlin Hayden, a spokeswoman for Obama's national security council, indicated the administration would consider acting unilaterally. "The US will continue to consult with the UK 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 US appears to have taken British support for granted. Hours before the vote, the chairman of the Senate intelligence committee, Diane Feinstein, expressed confidence that Britain would join any strike. Feinstein, a Democrat and staunch administration ally, told Time magazine: "I think the UK makes a difference. I think if the president were to decide to go there's a very high likelihood that the United Kingdom would be with us." The timing of the British vote, 272 to 285 against the government was disastrous for Obama. Less than 30 minutes after the vote, senior intelligence officials began a conference call with key members of Congress, in an attempt to keep US lawmakers on side. Congressional leaders and the chairs and ranking members of national security committees were briefed by the most senior US intelligence officials, amid signs that some of the support for military strikes against Syria was fading. The officials said there was "no doubt" that chemical weapons were used in Syria last week, Reuters reported. Obama aides cited intercepted communications of Syrian officials and evidence of movements by Syria's military around Damascus before the attack that killed more than 300 people, said Eliot Engel, the top Democrat on the House foreign affairs committee. The 90-minute briefing was conducted by secretary of state John Kerry, secretary of defense Chuck Hagel, national security adviser Susan Rice, among others. After the briefing, Carl Levin, the Democratic chairman of the Senate armed services committee, urged a cautious approach. "I have previously called for the United States to work with our friends and allies to increase the military pressure on the Assad regime by providing lethal aid to vetted elements of the Syrian opposition."