Syrian diplomats to be expelled from UK
Three senior Syrian diplomats are to be expelled, the Foreign Office has confirmed.
Others including France, Germany and Australia are also expelling diplomats in a coordinated protest against the killing of civilians.
The UN has said most of the 108 civilians killed in Syria's Houla region on Friday, including 49 children, were "executed".
The Syrian Ambassador left the UK earlier this year for health reasons.
The diplomats being expelled are the head of mission in the London embassy and two other senior figures.
The BBC's diplomatic correspondent Bridget Kendall said the Foreign Office wanted to send a stark message to the Syrian government in the wake of the Houla killings that it can not expect to act with impunity.
Foreign Secretary William Hague said the Syrian charge d'affaires, the most senior official left in the UK, had been asked to leave the country within seven days.
"Other Syrian diplomats, two other diplomats, will be expelled at the same time," he added. "Our allies and partners around the world will be taking similar action.
"Of course we will seek other ways to increase the pressure - we are discussing in the EU a further tightening of sanctions."
France, Germany, Italy, Spain, the United States, Canada and Australia are among other countries also asking Syrian officials to leave.
The expulsions "expressed our horror at the behaviour" of the Syrian government, Mr Hague added.
"We are increasing the pressure on the Assad regime and getting the message across that the international community is appalled by the murder of so many innocent people.
"Time will run out for the Annan plan and they (the Syrian regime) need to make a choice of what they are going to do."
Labour said they supported the expulsions as a way of "expressing the revulsion widely felt here and abroad about the Assad regime's conduct".
But shadow foreign secretary Douglas Alexanader added: "The actions so far agreed by the international community since the massacre last weekend have alas not yet matched the seriousness of the situation."
The diplomatic action came as UN-Arab League envoy Kofi Annan held talks with Syrian President Bashar al-Assad in Damascus.
According to Syrian state television, President Assad told Mr Annan that terrorists had stepped up their operations across Syria, including killing and kidnapping.
The UK has said the Syrian authorities must implement Mr Annan's plan which calls - among other things - for an end to violence by all sides, the freeing of political detainees and for aid agencies to be given access to Houla and other areas.