Britain is to put a resolution to the United Nations Security Council later on Wednesday "authorising necessary measures to protect civilians" in Syria.
The resolution will be put forward at a meeting of the five permanent members of the council, Prime Minister David Cameron said on Twitter, the BBC reports.
Mr Cameron is convening a meeting of his National Security Council to discuss military intervention in Syria.
He has also spoken again to US President Barack Obama as the allies prepare a response to chemical weapons use near the Syrian capital Damascus
Earlier a team of UN weapons inspectors resumed work for a second day, probing the attack on 21 August that killed hundreds of children, women and men.
UN Secretary General Ban Ki-moon called on the Security Council to act together. He also says it is essential to establish the facts of what happened
The joint UN-Arab League envoy to Syria, Lakhdar Brahimi, says he's urged the United States to delay military action until it has been clearly established what happened outside Damascus last week.
Mr Brahimi said: "It does seem clear that some kind of substance was used ... that killed a lot of people" on 21 August.
But he emphasised that any military action needed Security Council authorisation.
United States vice president Joe Biden is the most senior US official so far to accuse the Syrian Government of using chemical weapons against its citizens.
But Mr Brahimi has called on Washington as well as Russia -- which says the rebels were responsible for the attack - to provide evidence to the UN, the BBC reports.
Russia and China have previously vetoed resolutions critical of Syria and may block any text deemed to approve military action.
Both countries and Iran have previously warned against launching an attack on the war-ravaged country, where more than 100,000 people are thought to have died in two years of fighting, says the BBC.