Just a day after recalling Britain's parliament to vote on how to respond to Syria's suspected use of chemical weapons, Cameron was ambushed when the opposition Labour party said it wanted greater parliamentary scrutiny and rebel lawmakers in his own ruling Conservative party said they would oppose him.
Earlier on Wednesday, UN Secretary General Ban Ki-moon had sought more time for inspectors to complete their work, Russia had said it was premature for air strikes and tabled a UN resolution, and the Labour party had made it clear it wanted clear proof that the Syrian government had used chemical weapons.
Cameron's failure to execute his original plan of action could hamper efforts by the United States to deliver a swift cruise missile strike against Syria as early as this week, potentially harming London's alliance with Washington.
Inspired by the legacy of public mistrust left behind by former Prime Minister Tony Blair's contested decision to go to war in Iraq in 2003, Labour leader Ed Miliband and some rebel Conservatives used the prospect of a government defeat in parliament to force Cameron to delay action.
After hours of impromptu negotiations between Cameron's political managers and the opposition, his office agreed that the United Nations Security Council should see findings from chemical weapons inspectors before it responded militarily.
"The United Nations Security Council must have the opportunity immediately to consider that briefing (from inspectors) and ... every effort should be made to secure a Security Council Resolution backing military action before any such action is taken," a British government motion to be debated in parliament on Thursday said.
Britain had previously declined to say it would wait for a UN report before launching military action.
But on Wednesday it promised parliament would be given a second vote before Britain committed to direct military action, rather than the single vote - on Thursday - it had initially promised.
"A watered-down motion is better than a defeat for the government," said Andrew Bridgen, a Conservative lawmaker who opposed immediate military action, told the BBC.
"The motion ... will not allow military action. We need hard evidence" to sanction military action.
UK lawmakers will still debate Britain's response to the chemical attack in Syria today after Cameron cut short his holiday, recalled Parliament, and rushed back to London to chair a meeting of the National Security Council.
Public support for action is fragile. A YouGov poll published showed 50% of the British public opposed a missile strike on Syria, with just 25% in favour of it.
Syria prepares for attack
Meanwhile, Syria appears to have evacuated most personnel from army and security command headquarters in central Damascus in preparation for a Western military strike, residents and opposition sources said on Wednesday.
Army units stationed near the capital have confiscated several trailer trucks, apparently to transport heavy weaponry to alternative locations, though no significant movement of military hardware has been reported, possibly due to heavy fighting near major highways, one of the sources added.