There are unconfirmed reports that the spokesman of the Syrian foreign ministry has defected.
Wael Merza, Syrian National Coalition presidential adviser, said that Makdissi had arrived in London. a claim Al Jazeera could not confirm.
Lebanons' Al Manar television station reported that Makdissi had been sacked for making statements that do not reflect the state's line.
"We know that in late July, he made comments saying that Syria was not going to use its chemical weapons, admitting, basically, to Syria having chemical weapons," Al Jazeera's Rula Amin reported from Beirut.
"Then there was a lot of effort by the goverment to try to defuse the statement," she said.
Makdissi's role had been scaled back since then, with fewer statements and press conferences.
"We've heard, from different sources, that he did leave Syria - at least he has not been in Syria for the past two days - through Lebanon and is now in London." our correspondent said, adding that British officials are neither confirming nor denying Makdissi's defection.
Makdissi often spoke at news conferences in Damascus to set out Syria's response to the uprising against President Bashar al-Assad.
In June of 2011, the Syrian government also fired spokeswoman Reem Haddad for saying that Syrians were seeking shelter in Turkey in order to visit their relatives there.
Hisham Jaber, director of the Middle East Centre for Studies and Research, told Al Jazeera that he believes Makdissi was given permission to visit his family in Lebanon and that he flew to London from Beirut.
"I think he had a reason, maybe, to leave the country, and he made his choice," he said.
"Of course the gentleman is not in the position to have a lot of secrets, but he was very close to the regime and has a lot of information. He's also a very brilliant person and can use his skills among the opposition."
There is no official confirmation on Makdissi's status from the Syrian government.