Syria's charge d'affaires has left his London post because he is "no longer willing to represent a regime that has committed such violent and oppressive acts".
The Foreign Office said Khaled al Ayoubi's decision illustrated the "revulsion and despair" the regime's actions were provoking amongst Syrians.
His defection comes as the battle between government forces and opposition fighters in Syria's largest city intensified , with thousands of civilians forced to flee to neighbouring countries.
Fierce fighting for the past several days has centred on the Salaheddine district in southwest Aleppo, where President Bashar al Assad's troops have been backed by helicopter attacks.
In London, a spokesperson for the FCO said: "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."
Sky News' Foreign Affairs Editor, Tim Marshall, said the defection was a serious one for Mr Assad.
"It is yet another chip, as they chip away at the edifice of the regime, which now no longer has cracks in it, it has gaping holes in it," he said.
A series of envoys from Damascus have now fled the regime, including Nawaf Fares, the country's ambassador to Iraq, who left for Qatar this month after publicly renouncing his post.
The Syrian diplomats who were stationed in the UAE and Cyprus have also fled the regime .