MOSCOW — A Russian organized crime whistle-blower who died jogging near his home in a London suburb six years ago, raising suspicions of an assassination, “more likely than not” died of natural causes, a British coroner ruled on Wednesday.

The whistle-blower, Alexander Perepilichny, 44, had been in apparent good health before collapsing during his run in Weybridge, southwest of London. Suspicions arose because he had been a witness in a high-profile and politically delicate case that implicated senior Russian officials. In 2015, three years after his death, a botanist reported having found a rare plant poison in Mr. Perepilichny’s preserved stomach contents.

Rare poisons were the weapon of choice in the assassination and attempted assassination of two other Russians living in Britain, both former security service agents, and the British government directly implicated Moscow in those deaths.

Mr. Perepilichny, a banker, had become a witness in a financial fraud case that led to the enactment of a United States sanction law targeting Russia, the Sergei L. Magnitsky Rule of Law Accountability Act. Mr. Perepilichny’s role had been to hand over to Swiss authorities wire-transfer records related to the fraud.