Britain is to seek UN Security Council backing for “all necessary measures to protect civilians” in Syria, Prime Minister David Cameron said.
A UK draft resolution will be put to a meeting of the five permanent members in New York today “condemning the chemical weapons attack by Assad”.
The Prime Minister said: “We’ve always said we want the UN Security Council to live up to its responsibilities on Syria. Today they have an opportunity to do that.”
Mr Cameron announced the move as he prepared to chair a meeting of the national security council (NSC) at No 10 to discuss the UK’s involvement in any military response.
Labour has made a fresh effort to make UN backing for a military intervention one criterion for voting with the Government when Parliament is recalled tomorrow to discuss the UK’s response.
A senior Labour source described the draft resolution as “one necessary step”.
“Ed Miliband made clear to David Cameron last night the importance of evidence from weapons inspectors and consideration of any action by the UN Security Council,” the source said.
“We will continue to scrutinise any proposed action to ensure there is a proper legal base.”
Labour leader Mr Miliband indicated yesterday that his party would consider supporting international action, “but only on the basis that it was legal, that it was specifically limited to deterring the future use of chemical weapons, and that any actions contemplated had clear and achievable goals”.
A team of UN weapons inspectors was back at the site of the attack on the outskirts of the capital Damascus today but they are able to look only at whether chemical weapons were used, not at who deployed them, and action could be taken before they have concluded their work.
Washington has said it will release further evidence of the regime’s culpability - expected to involve intercepted signals intelligence..
Russia, which does not accept that there is evidence the regime was behind the deadly attack, is almost certain to use its veto.
Foreign minister Sergei Lavrov warned that military action “will lead to the long-term destabilisation of the situation in the country and the region”.
China, which also has a permanent seat along with the US, UK and France, has also been consistent in blocking previous anti-Assad resolutions.
UN secretary general Ban Ki-moon urged the members of the Security Council to act.
“The body entrusted with international peace and security cannot be missing in action,” Mr Ban said.
“The Council must find the unity to act. It must use its authority for peace. The Syrian people deserve solutions, not silence.”
A Downing Street spokeswoman said: “Britain has drafted a resolution condemning the attack by the Assad regime, and authorising all necessary measures under Chapter 7 of the UN Charter to protect civilians from chemical weapons.
“The resolution will be put forward at a meeting of the five permanent members of the Security Council later today in New York.”
The Prime Minister hopes it will help persuade MPs to support the Government when they are brought back early from their summer break tomorrow to debate the Syria crisis.
He has warned that the world cannot stand idly by and must take firm action to show that the use of chemical weapons - in Syria or anywhere else - cannot be tolerated.
But he faces opposition to intervention from a number of his own backbenchers and polling shows the public is deeply reluctant for the UK to become embroiled in military action.
Former military chiefs have also issued stark warnings about the direction Mr Cameron is taking, warning that even a “surgical” missile strike could end up dragging the UK into deeper action.
And the Archbishop of Canterbury has urged MPs not to rush their decision, warning of the “unforeseeable ramifications”.
Mr Cameron has insisted that any intervention in Syria would not be about the conflict itself, but preventing the use of chemical weapons by any regime and would be “proportionate, have to be legal, would have to specifically be about deterring the use of chemical weapons”.
He held fresh talks with US president Barack Obama last night, after which No 10 said they were “in no doubt that Bashar Assad’s regime was responsible for the toxic assault”.
“Both leaders agreed that all the information available confirmed a chemical weapons attack had taken place, noting that even the Iranian president and Syrian regime had conceded this.
“They both agreed they were in no doubt that the Assad regime was responsible,” a spokesman said.
“Regime forces were carrying out a military operation to regain that area from the opposition at the time, and there is no evidence that the opposition has the capability to deliver such a chemical weapons attack.”
US defence secretary Chuck Hagel has indicated that US forces “are ready to go” when Mr Obama gives the word.
While political momentum towards intervention mounts, the British public has yet to be persuaded.
A YouGov survey for the Sun revealed that nearly three-quarters of people oppose the deployment of British troops to Syria, and a majority of 3-1 believe the Government should be bound by Parliament’s vote tomorrow.
Shadow foreign secretary Douglas Alexander told Sky News: “All of us, of whichever political party, are acutely aware of the experience of Iraq and the facts that mistakes were made on exactly these types of issues.
“Now, there is a very clear distinction. Very few people and certainly no credible people that I’ve heard dispute that there was a chemical weapons attack, or indeed a very significant attack, last week.
“The character of that attack is for the weapons inspectors to set out but that is a different set of circumstances from circumstances in 2003 where it emerged that those weapons of mass destruction didn’t exist and there was faulty intelligence.
“Very few people doubt that chemical weapons have been used but it is critical, not least in light of that experience, that the right processes are followed and the right processes are seen to be followed, not just by the public but by the international community.”