Britain will put a resolution to the United Nations Security Council later on Wednesday "authorising necessary measures to protect civilians" in Syria, David Cameron has said.
The prime minister announced the move on his Twitter feed on Wednesday morning. "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," he said.
"Britain has drafted a resolution condemning the chemical weapons attack by Assad & authorising necessary measures to protect civilians. The resolution will be put forward at a meeting of the five permanent members of the Security Council later today in New York."
Cameron has said the use of chemical weapons in Syria "is wrong" and that "any response wound have to be legal, proportionate and designed to deter further outrages".
The prime minister's revealed the decision as Ed Miliband qualified his support for any military strikes against the Bashar Assad regime on fresh efforts being made to secure UN backing.
The Labour leader had initially indicated he was prepared to side with the coalition in tomorrow's Commons vote. However this morning a party source said: "As part that legal justification, Labour is seeking the direct involvement of the UN through evidence from the weapons inspectors and consideration by the security council."
The resolution is highly unlikely to be passed given opposition from Russia and China.
Senior British ministers including Cameron, Nick Clegg and William Hague are due to meet at the National Security Council (NSC) in London today to discuss the international response.
Downing Street said today that both Cameron and President Obama are in no doubt that Assad's regime was responsible for the toxic assault on the outskirts of Damascus on August 21.
A No 10 spokesman said: "The PM spoke to President Obama last night to further discuss the serious response to last week's chemical weapons attack in Syria.
"Ahead of today's NSC, it was an opportunity for the PM to hear the latest US thinking on the issue and to set out the options being considered by the Government.
"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.
"And they both agreed they were in no doubt that the Assad regime was responsible. 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.
"The PM confirmed that the government had not yet taken a decision on the specific nature of our response, but that it would be legal and specific to the chemical weapons attack."
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 said: ''Let me stress to people, this is not about getting involved in a Middle Eastern war or changing our stance in Syria, or going further into that conflict. It's about chemical weapons. Their use is wrong and the world should not stand idly by."