Jihad al-Makdissi, a Syrian foreign ministry spokesman and one of the most public faces of Bashar al-Assad's government, has reportedly fled the country.
Syria spokesman 'flees country' (Reuters)
A diplomat in the region said that al-Makdissi had defected.
"He defected. All I can say is that he is out of Syria," the diplomat, who did not want to be named, told Reuters, adding to reports Channel 4 News has had from sources that al-Makdissi has been "exempted" from office and has left the country.
Al-Makdissi, who is in his 40s, previously worked at the Syrian embassy in London and returned to Damascus a year ago to serve as spokesman for the ministry, defending the government's crackdown on the revolt against Assad's rule.
While al-Makdissi had little influence in a country largely run by the military and security forces, his defection will be seen as a blow for President Assad as his government continues to battle with rebels in a 20-month uprising.
Some reports from the region suggested that al-Makdissi, who speaks fluent English, had been sacked for making statements that did not reflect the government's position.
A UK Foreign Office spokeswoman said: "We're aware of reports that he has defected and may be coming to the UK. We're seeking clarification."
Al-Makdissi's apparent defection comes as heavy fighting continued in recent days, with rebels seizing military bases in recent weeks including some close to the capital Damascus.
Watch: Is Syria's civil war reaching a turning point?
The conflict, which has already seen 40,000 people killed, has grown increasingly bloody in recent months as rebels moved on Damascus.
On Monday, US President Barack Obama warned President Assad against using chemical weapons.
"The world is watching," Obama said.
"The use of chemical weapons is and would be totally unacceptable and if you make the tragic mistake of using these weapons there will be consequences and you will be held accountable."
However, Syria's foreign ministry stressed that it would not use chemical weapons against its own people.
Also on Monday, the United Nations said it was withdrawing "all non-essential international staff" from Syria as a result of deteriorating security. The European Union is also pulling out its international staff and its ambassador.