Foreign Secretary William Hague has said the UK has decided to recognise the Syrian opposition coalition.
He told MPs the National Coalition of the Syrian Revolutionary and Opposition Forces was the “sole legitimate representative” of the Syrian people.
He said they were now a “credible” alternative to the Assad government.
In the absence of a diplomatic solution, he told MPs the UK would not rule out any action - subject to international law - to save lives.
Up to 30,000 people have been killed in the 18-month conflict, the UK believes.
Those trying to bring down President Bashar al-Assad’s government moved closer to a united front when the rival leaders of Syria’s rebels formed the coalition after months of bitter division.