There are fears that the death of Badri Patarkatsishvili, 52, might plunge his native Georgia into further political turmoil.
The wealthy businessman had become an outspoken opponent of President Mikheil Saakashvili, and financed his own campaign in January's early presidential ballot.
Georgia's political atmosphere is still highly charged in the wake of a state of emergency ahead of that vote and pressure on Saakashvili to be less confrontational toward political opponents, including rivals like Patarkatsishvili.
Georgians awoke on February 13 to a flurry of reports announcing Patarkatsishvili's passing. The reports said he had been struck by a heart attack at his mansion near London.
Patarkatsishvili had moved to the British capital in November after Georgian authorities accused him of trying to overthrow President Saakashvili -- against whom he ran in a snap election in January -- and issued a warrant for his arrest.
Little is known so far about the circumstances surrounding his death, which like all sudden deaths was being investigated by the British police as "suspicious." A postmortem examination was expected to be carried out later in the day.
Members of Saakashvili's ruling National Movement party were initially refraining from comment. "I can't tell you anything other than the official version, since I don't know any details," lawmaker Nino Nakashidze said. "We have to wait; experts will probably release their conclusions. The official version, which is all we know now, is a heart attack."
But some of Patarkatsishvili's allies have been quick to pin the blame for his heart attack on the legal charges brought against him by Georgian authorities, and have called for an international inquiry.
Allies Question Early Reports
Patarkatsishvili was a driving force behind massive antigovernment protests in November, which were extensively broadcast by the independent television station Imedi that he partly owned. But he had strongly denied accusations that he planned to topple Saakashvili's regime.
He had acknowledged, however, offering large sums of money to police not to break up the protests, which were eventually forcefully dispersed and followed by a nationwide state of emergency.
Patarkatsishvili's battle with the Georgian government have led some of his supporters to cast cautious doubt on the initial reports citing a heart attack as the cause of death.
"We don't know anything much right now --saying anything would be giving out false information," said Zaur Kirkitadze, a member of Patarkatsishvili's election campaign team. "We know nothing -- only the fact that the man is dead, supposedly from heart failure. But we also know that he did not suffer from any heart condition."
Davit Shukakidze, who also worked on Patarkatsishvili's presidential election campaign, told Reuters he suspected foul play.
"The medical check will be conducted and we will find out what caused the death, but in any case it was murder, the murder which was committed by this government," Shukakidze said.
Patarkatsishvili himself had repeatedly hinted at threats against his life. On December 27, he briefly pulled out of the presidential election, saying his life was in danger. The same month, he told AP that he had obtained a video showing a Georgian Interior Ministry official commissioning a Chechen warlord to kill him in London.
Friend and longtime business partner Boris Berezovsky saw Patarkatsishvili hours before his death
Patarkatsishvili discussed his claims in a December interview with RFE/RL's Russian Service.
"When all this started -- not just this tape, but their resistance against me in general -- I did not think it would come to this," Patarkatsishvili said. "But then I acquired big amount of evidence, which makes it possible for me to say that, in reality, the present Georgian government is planning to kill me with all possible means."
In October, former Defense Minister Irakli Okruashvili stunned Georgians by alleging in a televised statement that Saakashvili had commissioned him to assassinate Patarkatsishvili. Okruashvili was arrested and later retracted the statement.
An Oligarch's Life
Patarkatsishvili, who earned his fortune in Russia during the 1990s, was an ally of Russian business tycoon Boris Berezovsky, who has himself lived in London for several years in self-imposed exile.
Berezovsky told news agencies he had visited Patarkatsishvili several hours before his death. In an interview with RFE/RL, he said he was waiting for the police to conclude their investigation before he offered further comment.
"Badrik passed away yesterday late in the evening," Berezovsky said. "Police [are] working there now, and only they can evaluate what has actually happened."
Whatever killed Patarkatsishvili, his ties to Russia are also likely to inspire comparisons with the case of Aleksandr Litvinenko, a former Russian security service officer and Kremlin critic who died of radioactive poisoning in London in 2006.
Litvinenko, who had been granted British citizenship, in a deathbed statement accused Russian President Vladimir Putin of commissioning his assassination.
Aleksei Malashenko, a Moscow-based political analyst, says it is only a matter of time before Russia is blamed for having a hand in Patarkatsishvili's death.
"It may be a coincidence, but things happen suspiciously often to Russians or to people connected with Russia in Britain," Malashenko said. "This card will definitely be played -- I can assure you that some newspapers tomorrow will accuse Moscow of murdering Patarkatsishvili."
Malashenko added that "Russia's reaction will play an important role and hopefully will not further strain relations with Britain."
British officials have strongly criticized Russia over its unwillingness to extradite Andrei Lugovoi, a former KGB operative and former security chief for Berezovsky and Patarkatsishvili at Russian ORT television whom he British prosecutor's office has accused of murdering Litvinenko.
Patarkatsishvili was wanted by Russia on charges of embezzling funds from the country's largest car manufacturer, AvtoVAZ, and conspiring to free a Berezovsky associate from police custody.
Despite his murky resume, Malashenko predicts the death of the flamboyant, mustachioed tycoon will further Georgia's political woes at a time when Saakashvili is struggling with credibility issues and facing off against an increasingly strong opposition.
"He undeniably played a positive role," Malashenko said of Patarkatsishvili. "At any rate, he was a counterbalance, he was an influential, restrained man -- he wasn't a hysteric. I think a vacuum will appear on Georgia's political map."
He used a chess analogy in reference to Patarkatsishvili's influence on Georgian politics, saying, "he was an important piece -- a rook, and maybe even a queen."
(RFE/RL's Georgian and Russian services contributed to this report.)