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.
At a meeting last week, Britain urged the newly formed coalition of opposition groups to put forward a clear plan for political transition in Syria.
In a statement to Parliament, Mr Hague said the Syrian opposition would be invited to appoint a political representative to the UK and would be given £1.6m to help with communications equipment.
The UK will also offer assistance to help the coalition set up political and humanitarian structures and deploy a "stabilisation response" team in Syria to help provide basic services to people in opposition-controlled areas.
Mr Hague said the decision to recognise the opposition was taken after consultation with European partners and after its leaders offered "assurances" about their commitment to uphold human rights and work for a democratic future at Friday's meeting.
"It is strongly in the interests of Syria, of the wider region and of the United Kingdom that we support them and deny space to extremist groups," he added.
"A credible alternative to the Assad regime is emerging that has the growing support of the Arab League, the European Union, the United States and an increasing number of other countries."
A spokesman for the coalition, Walid al-Bunni, told Reuters the UK's move was "very important" and would "encourage more Syrians to join the coalition and trust it and it will also encourage other European states to recognise it".
France, Turkey and six Gulf states have already recognised the opposition although the US has stopped short of doing so.
President Barack Obama has described the coalition as "a legitimate representation of the aspirations of the Syrian people" but said he was not yet prepared to regard it as a "government-in-exile" and was seeking assurances that elements within the opposition were not hostile to US interests in the region.
The European Union has recognised the coalition as "legitimate representatives" of the Syrian people, but not the sole one.
For Labour, shadow foreign secretary Douglas Alexander welcomed the UK's decision, saying it was an appropriate response to the "new found unity" within the opposition movement.
"Recognition is a vital step forward," he said. "Only a credible and inclusive transition plan and a united opposition holds the prospect of being a bridge between conflict and a sustainable peace in Syria."
But he sought guarantees that the move was not a "precursor" to the UK arming the Syrian opposition, which he said would be against the EU arms embargo and detract from diplomatic and humanitarian efforts.
In response to a question from Conservative MP Sir Malcolm Rifkind - who has consistently called for the international community to arm Syrian opposition groups - Mr Hague said the government's position on military support had not changed.
"The arms embargo is due to be continued from 1 December as part of the entire package of Syria sanctions," he added. "We will keep all these options under review but we have made no decision to change our policy on arms supplies as things stand."