Just a day after recalling Britain's parliament to vote on how to respond to the alleged use of chemical weapons in Syria, Cameron was ambushed when the opposition Labor party said it wanted greater parliamentary scrutiny and lawmakers in his own ruling Conservative party said they would oppose him.

The Labor 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, Labor leader Ed Miliband and some Conservatives used the prospect of a government defeat in parliament to force Cameron to delay action.

"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.

Hundreds of people were killed and scores of others were injured in a chemical attack in the Damascus suburbs of Ain Tarma, Zamalka and Jobar on August 21.

The Damascus government said the chemical attack was carried out by the militants.

NTJ/NJF