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.
Hague is probably not the sort of conservative I agree with often, but here I see him as correct.
Palestinian Islamist group Hamas “bears principal responsibility” for escalating tension in Gaza and southern Israel, UK Foreign Secretary William Hague has said.
In a statement, Mr Hague said he deeply regretted “the loss of civilian life on both sides”.
But he added: “I utterly condemn rocket attacks from Gaza into southern Israel by Hamas and other armed groups.”
Three people were killed when rockets fired from Gaza struck southern Israel.
Israel killed Hamas’ military chief in Gaza on Wednesday.
Rocket attacks from Gaza create “an intolerable situation for Israeli civilians in southern Israel, who have the right to live without fear of attack” and “also risk worsening the plight of Palestinian civilians in Gaza, which is already precarious”, he said.
Foreign Secretary William Hague has withdrawn all diplomatic staff from the British Embassy in Damascus in Syria and suspended its services.
Mr Hague said the deteriorating security situation put staff in danger.
The move comes as the UN rights council condemned “systematic violations” against civilians by the Syrian regime.
Prime Minister David Cameron said the UK is taking steps to build a legal case against President Bashar al-Assad for violently supressing protests.
He said Britain is making sure that experts are available on the Turkish border and elsewhere to collect the evidence.
“There will be film evidence, there will be testimony, there will be individuals, there may well be information from the cameraman who recently escaped from Homs,” he said.
The British prime minister said the aim was to “build a picture that can then make a case that these are crimes against humanity” and that the Syrian president “must be held to account”.
“I wish we could do more but we have to be realistic about what we can achieve. But holding them to account, gathering the evidence, using that case to build a case in international law that he can never hide from - that we can do,” said Mr Cameron.
It comes after he said the international community was exerting maximum pressure on the regime.
Mr Cameron has said Britain must be realistic about its capacity to intervene directly, insisting the situation was different to Libya.
Meanwhile, the British Red Cross has launched a “crisis appeal”, saying Syria urgently needs aid.