Alexander Perepilichny's body was discovered in November 2012 outside his rented house in southern England.
He had been in possession of documents that allegedly blew the lid off a massive Russian tax fraud involving dirty money being funneled into Swiss bank accounts.
There were fears the 44-year-old had been poisoned, and two autopsies carried out in November failed to establish the cause of death.
Police said today that toxicology samples taken from both autopsies came back clear and that the matter has been formally passed to the coroner for an inquest.
The force acknowledged frustration about the length of time it took to complete the inquiries, stressing that it was important to rule out foul play and put to rest speculation he had been poisoned, a possibility floated given the suspicious deaths of other Russians in the UK.
Perepilichny had been aware of reports that his life had been threatened, and was cooperating as a whistleblower in relation to the same money laundering scheme that was being investigated by Sergei Magnitsky, a Russian lawyer hired by the London-based hedge fund Hermitage Capital, who died in a Moscow jail in 2009 amid torture claims.
