Alexander Perepilichny, 44, was found dead near his luxury home on the exclusive gated St George's Hill estate in Weybridge, Surrey, southwest of London, after he had been out jogging in November 2012.
The sudden nature of the death of Perepilichny, who had sought refuge in Britain in 2009, and his role in helping a Swiss investigation into a Russian money-laundering scheme sparked speculation that he might have been murdered.
"I am satisfied on the evidence I have heard I can properly and safely conclude that it was more likely than not that he died of natural causes, namely sudden arrhythmic death syndrome," Coroner Nicholas Hilliard said.
"There really is no direct evidence that he was unlawfully killed."
Hilliard said an unidentified compound found in his stomach had no link to the gelsemium plant species and was widely occurring.
No further tests were possible because detectives had flushed away his stomach contents, the inquest was told.
The coroner said London police had contacted him this month to confirm they were not conducting an investigation into his death and that there was no evidence of "any hostile state actor" being involved.
Perepilichny's death was re-examined after the nerve agent attack in March on ex-Russian spy Sergei Skripal and his daughter in Salisbury, southern England, which Britain blamed on Moscow. Russia denied any wrongdoing.
His widow, Tatiana, told the inquest she did not believe her husband had been murdered. (Editing by Andrew MacAskill and Guy Faulconbridge)
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