Syria's most senior diplomat in the UK has quit his post in protest at the "violent and oppressive" actions of the regime of president Bashar Assad, the Foreign Office said.
In a blow to Damascus, charge d'affaires Khaled al-Ayoubi informed the British authorities that he was "no longer willing" to represent the government amid the brutal repression of rebels.
The British Government said his decision represented the "revulsion and despair" of Syrians and urged others around the world to follow his example and abandon Assad.
An FCO spokesman said: "Mr al-Ayoubi has told us that he is no longer willing to represent a regime that has committed such violent and oppressive acts against its own people, and is therefore unable to continue in his position.
"Mr al-Ayoubi was the most senior Syrian diplomat serving in London. His departure is another blow to the Assad regime. It illustrates the revulsion and despair the regime's actions are provoking amongst Syrians from all walks of life, inside the country and abroad.
"We urge others around Bashar Al-Assad to follow Mr al-Ayoubi's example, to disassociate themselves from the crimes being committed against the Syrian people and to support a peaceful and free future for Syria."
The Syrian regime has been hit by a series of defections, including three high-ranking diplomats and a number of military commanders.
A Foreign Office spokesman said it did not yet have any confirmation that Mr al-Ayoubi would transfer his support to the rebels. He was staying in a safe location in the UK and was in contact with British officials, he said.
Mr al-Ayoubi's departure leaves five staff at the Syrian embassy after his predecessor and two others were expelled at the end of May. There had so far been "no indication that they are leaving their posts", the spokesman said.
The announcement came after the United Nations reported that 200,000 Syrians had fled Syria's largest city, Aleppo, where government forces have been bombarding rebels for 10 days in the latest chapter of the 18-month uprising. Those fleeing the violence have described incessant shelling, constant gunfire, food shortages and power cuts.