Britain has ordered Syria's charge d'affaires to leave the country within seven days as part of a co-ordinated global response to a massacre of 108 people in Houla.
The United Nations said most of the victims last week were shot at close range, some of them women, children and entire families gunned down in their own homes.
Foreign secretary William Hague expressed horror at the events and alongside similar action France, Germany, Italy, Spain, the United States, Canada and Australia ordered three diplomats, including the charge d'affaires out of the country.
Mr Hague said: "All our efforts are going into supporting the Annan plan to try and bring about a peaceful transition and increasing the pressure on the Assad regime to implement that plan.
"These expulsions express our horror at the behaviour of the regime and increase international pressure to implement the commitments they have entered into."
The United Nations has been looking into Friday's massacre in a bid to establish exactly what happened.
A report from the human rights office indicated most of the dead were killed execution-style, with less than 20 people killed by regime shelling. The UN reported survivors and witnesses blaming the house-to-house killings on pro-government thugs known as shabiha, who reportedly often operate as hired muscle for the regime.
Rupert Colville, spokesman for the UN High Commissioner for Human Rights, said: "What is very clear is this was an absolutely abominable event that took place in Houla, and at least a substantial part of it was summary executions of civilians, women and children.
"At this point, it looks like entire families were shot in their houses."
The Syrian regime has denied taking part in the massacre, instead blaming the killings on armed terrorists.
According to the state-run news agency, Sana, President Bashar Assad blamed terrorists and weapons smugglers for scuttling the peace plan. The regime denies there is any popular will behind the country's uprising, saying foreign extremists and terrorists are driving the unrest.
Deputy foreign minister Faisal Mekdad said: "It is irrational that any party who wants to make Annan's mission a success would ever commit such a massacre."
He added the government had not "committed a single violation" of the plan.
In a statement explaining the expulsions of diplomats from the US, the State Department described the massacre as a "vicious assault involving tanks and artillery - weapons that only the regime possesses".
Spokeswoman Victoria Nuland said: "There are also reports that many families were summarily executed in their homes by regime forces.
"We hold the Syrian government responsible for this slaughter of innocent lives."
Australian foreign minister Bob Carr said: "This is the most effective way we've got of sending a message of revulsion of what has happened in Syria."
The Russian government continued to offer qualified support to the Assad regime, though it has been critical of events in Houla.
But the Russian foreign minister Sergey Lavrov also warned of unnamed countries trying to use the Houla killings "as a pretext for taking military measures".
Syria has seen unrest for more than a year after protests began in March 2011, alongside uprising and revolutions in other Arab and North African countries.
Former UN secretary general Kofi Annan brokered a ceasefire and a six point plan earlier this year following prolonged fighting in Homs.
But last week's Houla killings prompted sweeping international condemnation and the harshest language yet from Syrian ally Russia - making it a potential turning point in the crisis that has killed more than 9,000 people.
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