Georgian businessman and political activist Badri Patarkatsishvili died of natural causes, according to interim post mortem results, but extensive toxicological tests have not yet been carried out, British police said Thursday.

Patarkatsishvili, 52, died at his home in Leatherhead, south of London, on Tuesday.

"Following initial inquiries and the post mortem carried out last night, Surrey Police can confirm that at this stage there is no indication that the sudden death of Badri Patarkatsishvili was from anything other than natural causes," the department said in a statement.

"However, extensive toxicology testing is yet to be carried out. This will take a number of weeks."

A coroner's inquest will be opened on Friday, police added.

Patarkatsishvili had built his fortune in Russia after the breakup of the Soviet Union through a partnership with fellow tycoon Boris Berezovsky, one of Russian President Vladimir Putin's most vocal foes.

Campaigning from Britain, he ran for president last month and lost to incumbent Mikhail Saakashvili. Opposition groups say the vote was rigged.

In Georgia, Patarkatsishvili's spokesman, Guga Kvitaishvili, said the cause of death apparently was heart failure.

Family members and other unidentified people were with Patarkatsishvili when he collapsed around 11 p.m. Tuesday in his home 20 miles (30 kilometers) south of London, Surrey Police said.

Berezovsky said relatives told him the opposition leader died of a heart attack.

"I have lost my closest friend," Berezovsky said.

Berezovsky told the AP that Patarkatsishvili had not been ill, but had complained about his heart when they met Tuesday. "I saw him yesterday," Berezovsky said.

Patarkatsishvili told The Associated Press in December that he had obtained a tape recording of an official in his country's Interior Ministry. He said the official was heard asking a Chechen warlord to murder him while he was in London.

"I believe they want to kill me," Patarkatsishvili told the AP. It was not possible to verify his claim.

In a Dec. 23 statement released through the Bell Pottinger public relations firm, he demanded Georgian police investigate what he called the "plot to assassinate me."

"If the authorities fail to respond to this urgent appeal and do not take appropriate steps, they will be held responsible for that," he said in the statement.

Scotland Yard said at the time the Georgian had not contacted British police about any plot to kill him. On Wednesday, Scotland Yard said they would not discuss the alleged threat.

Patarkatsishvili helped lead anti-government protests in November and was under investigation at home on charges of plotting to overthrow the government.

He denied the accusations, but acknowledged offering a senior police official US$100 million (euro69 million) if police agreed not to respond with force during street protests after the January election.

Patarkatsishvili left Georgia in November and lived in self-imposed exile in Britain and Israel.

Patarkatsishvili placed third in last month's election with 7 percent of the vote. Saakashvili won with 53 percent.

Among Patarkatsishvili's businesses was the Imedi television station, which is managed by Rupert Murdoch's News Corp. The station had been critical of Saakashvili's government. The station is closed, at least temporarily.

Patarkatsishvili became Berezovsky's business partner in 1989. During the major privatizations of the 1990s, the pair invested in oil, airlines and the auto industry.

Patarkatsishvili was responsible for their media holdings, including national television network ORT.

However, the pair claimed in British court documents that the Kremlin forced them to sell their stakes in oil company Sibneft, Russian Aluminum and ORT for a fraction of their value. Berezovsky and Patarkatsishvili made nearly US$1 billion each from the sale.

Patarkatsishvili fled from Russia to Georgia in 2001 after he was accused of helping a colleague try to break out of prison and after Berezovsky fell out with Kremlin. Patarkatsishvili denied the charges and claimed the Kremlin had targeted him in a crackdown on independent media.

He helped bankroll Georgia's "Rose Revolution," which brought down Eduard Shevardnadze's regime and brought Saakashvili to power in 2003. Patarkatsishvili and Saakashvili later fell out over policy differences.