Britain today recognised the fledgling Syrian National Coalition, joining France in saying the opposition group was the "sole legitimate representative of the Syrian people".
The announcement, by the country’s Foreign Secretary, William Hague, could boost the group's chances of securing foreign aid and arms as it seeks to oust President Bashar al-Assad.
"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," Mr Hague told his parliament.
He said he believed other European Union nations would follow suit.
The Syrian National Coalition for Opposition and Revolutionary Forces, set up this month to unite diverse opposition voices, welcomed the move.
"This step is very important and I think it will encourage more Syrians to join the coalition and trust it and it will also encourage other European states to recognise it," the coalition's spokesman Walid al-Bunni said.
France was the first EU country to recognise the group, going further than Monday's EU-wide recognition of the coalition as the legitimate representatives of the "aspirations" of the Syrian people.
"It's a morale boost. It gives some credibility to the opposition, and it could lay the platform practically for more effective ways of channelling support, plus some quasi-military support," said David Butter, Middle East expert at London-based think-tank Chatham House.
The United States has pledged to work with the Syrian opposition coalition, but has stopped short of fully recognising it.
Turkey and Gulf Arab countries were quick to endorse the coalition, formed on 11 November in Doha.
Britain and France, along with the US, led the military campaign against Libyan former leader Muammar Gaddafi last year, and Paris said it was now considering whether to arm Syria's rebels.
Britain says no option is off the table, but Mr Hague told the House of Commons that no decision had been taken to supply military aid. The EU has an arms embargo on Syria, which is expected to be renewed on 1 December.
The West has been more wary in dealing with the Syrian opposition, due to concerns that some groups lack support on the ground and worries about Islamic radicals in the rebel ranks.
United Nations investigators have said rebel fighters may have committed war crimes.
Mr Hague said he has received assurances from the Syrian coalition that the mainly Sunni Muslim opposition would respect Syria's other minorities and uphold human rights.
Assad allies Russia and China have so far vetoed strong UN Security Council action against the Syrian president.
An estimated 38,000 people have been killed in Syria since an Arab Spring-inspired uprising against Assad's autocratic rule began in March last year. The initially peaceful protests turned into an armed rebellion after harsh government crackdowns.
Syrian government troops, backed by tanks, battled to oust rebel forces from an opposition stronghold in a Damascus suburb today in the heaviest fighting in the capital for months.
After months of slow progress marked by poor organisation and supply problems, the rebels have captured several army positions in outlying regions in the last week, including a Special Forces base near Aleppo, Syria's commercial hub.
They are also trying to take the 20-month-old revolt to the heart of Damascus, Assad's seat of power, and have dubbed this week "March to Damascus Week".
Elite Republican Guard troops attacked the rebel stronghold of Daraya on the city's southwestern edge but met resistance from rebel fighters of the Free Syrian Army, opposition sources said.
In total, 100 people were killed in violence today, 64 of them civilians, the pro-opposition Syrian Observatory for Human Rights said.