The Foreign Office warned that violence in Syria had reached a "new scale" amid claims that more than 300 people had been massacred.
Opposition activists said hundreds of bodies, including women and children, had been found in the town of Darayya, near the capital Damascus.
Many are alleged to have been summarily executed as Bashar Assad's regime attempts to stamp out dissent.
Foreign Office minister Alistair Burt said: "I am deeply concerned by emerging reports of a brutal massacre of civilians in Darayya, in the outskirts of Damascus.
"The Syrian regime's appalling repression of its people, over more than 17 months, has left little space for independent observers to operate in Syria.
"This makes it extremely difficult to verify what took place yesterday. Opposition groups report that over 300 people, including women and children, were killed and that some were shot at close range.
"If confirmed it would be an atrocity on a new scale, requiring unequivocal condemnation from the entire international community.
"It would make yesterday the bloodiest day since the unrest in Syria began in March 2011, with over 400 killed across the country."
Mr Burt said the development highlighted the urgent need for international action to bring an end to the violence. Russia and China have blocked UN Security Council resolutions that would increase pressure on the regime.
Meanwhile, Syrian Vice-President Farouq al-Shara has greeted an Iranian delegation in Damascus, ending weeks of speculation that he had defected.