The death of an exiled Georgian billionaire, who claimed less than two months ago that he was the target of an assassination plot, has been handed over to a major crimes investigation team.

Police have confirmed that they are treating the death of Badri Patarkatsishvili, 52, as "suspicious." The businessman, who was Georgia's richest man and a close friend of the anti-Kremlin oligarch Boris Berezovsky, died of an apparent heart attack at his $19,622,000 mansion in Leatherhead, Surrey, last night.

"As with all unexpected deaths it is being treated as suspicious. A post-mortem examination will be held later today to establish the cause of death," Surrey police said.

Patarkatsishvili, worth an estimated $11,773,201,054, funded an opposition campaign against Georgia's pro-Western leader Mikheil Saakashvili and stood against him in last month's presidential election. Georgia accused him of plotting a coup after airing a tape of him offering a $100 million bribe to a police chief to support opposition demonstrators.

He told The Sunday Times in December that Saakashvili's regime was planning to send an assassin to kill him in London. He released a covert tape recording of negotiations between what he said was a Chechen warlord and an official from the Georgian Interior Ministry.

Patarkatsishvili hired Lord Goldsmith, the former attorney general, to represent him as Georgian authorities mounted investigations into his business interests in the former Soviet republic.

Berezovsky said that his former business partner had complained about his heart when the pair met earlier on Tuesday, but had not been ill.

He said: "The death of Badri Patarkatsishvili is a terrible tragedy. I have lost my closest friend. This is a huge loss for all of his family and friends.

"I shall make no further comment on the circumstances of Badri’s death. I shall wait for the authorities to complete their investigation."

Patarkatsishvili lived in Russia between 1993 and 2001. In the 1990s, he was wanted by Russian authorities on charges of theft from the country's largest car factory, AvtoVAZ, which he ran with Berezovsky.

He was also accused of plotting to arrange the escape from custody in 2001 of Nikolai Glushkov, deputy director of Aeroflot, Russia's national airline, who had been accused of fraud.

The man charged with breaking out Glushkov was Andrei Lugovoy, who was arrested and jailed after the attempt failed. Lugovoy is wanted by the British Crown Prosecution Service for the murder of Alexander Litvinenko, the dissident former Russian spy poisoned in London with radioactive polonium-210 in 2006.

Lugovoy was responsible for protecting Patarkatsishvili and Berezovsky at the time as head of security at the Russian TV channel ORT, which the two men controlled.

Patarkatsishvili remained good friends with Lugovoy, a former KGB officer who is now a member of the Russian parliament. The pair were seen socializing together in Tbilisi, the Georgian capital, shortly before Litvinenko was poisoned.

Litvinenko also had links with the Georgian businessman. Sources in Tbilisi have told The Times that he stayed at Patarkatshvili's residence in Georgia en route to Turkey when he fled Russia to seek asylum in London in 2000.

Russian prosecutors claim that Litvinenko also visited Patarkatsishvili as well as Berezovsky in London shortly before he was poisoned. They accuse Berezovsky of involvement in the murder of the former Federal Security Service (FSB) agent as part of a plot to damage President Putin's international image.

Georgia's former Defense Minister, Irakli Okruashvili, accused Saakashvili of encouraging him to kill Patarkatsishvili in 2005, although he later retracted the claim.

The tycoon helped to finance the “Rose Revolution” that swept Saakashvili to power in Georgia in 2003, but the two men later fell out and he accused the president of turning into a dictator.

When Georgian police used tear gas and rubber bullets to disperse opposition street protests in November, special forces troops also stormed the studios of Imedi TV and forced it to shut down. Patarkatsishvili founded the station and News Corporation, which also owns The Times, was managing it at the time of the incident.