The Syrian charge d'affaires is being expelled from Britain amid mounting condemnation of attacks on civilians by the Assad regime.
Two other diplomats are also being told to leave but the embassy in London is not being closed.
The move, mirrored by several other western countries, follows more than a year of unrest and worldwide revulsion over the recent massacre at Houla.
Britain's diplomatic sanction against Syrian President Bashar Assad's regime came alongside similar moves by France, Germany and Australia.
International condemnation of the latest deaths mounted earlier when the United Nations said a large majority of the 108 victims - including children - were summarily executed.
Fewer than 20 died from artillery fire, the office of the UN High Commissioner for Human Rights reported - as international envoy Kofi Annan met President Assad for crisis talks.
Activists say the executions were carried out by pro-government forces but the regime continues to deny any involvement.
The charge d'affaires was summoned to the Foreign Office yesterday for a dressing-down over what political director Sir Geoffrey Adams described as "a sickening and evil crime".
Sir Geoffrey told the Syrian diplomat that Britain wanted all those responsible to be identified and held accountable and warned of "further quick and robust action" from the international community unless all military operations ceased immediately.