Boris Berezovsky, 67, was found dead by an employee on Saturday on a bathroom floor at his home in Berkshire.
The post-mortem examination found nothing to indicate a violent struggle. More tests are to be carried out on the body.
Thames Valley Police said investigations at the property would continue for several days.
They had earlier said there was no evidence so far that a “third party” was involved.
Boris Berezovsky’s body was removed from the property in Mill Lane, Ascot overnight on Sunday.
The results of further tests, including toxicology and histology examinations, were likely to take several weeks, police said.
Crime officers have been carrying out forensic examinations at the property and “these are expected to last several days”.
Boris Berezovsky’s body was reportedly found by an employee, who called an ambulance at 15:18 GMT on Saturday.
He had not been seen since around 22:30 GMT the previous evening.
Boris Berezovsky emigrated to the UK in 2000.
He amassed a fortune in the 1990s following the privatisation of state assets after the collapse of Soviet communism.
The Russian oligarch survived numerous assassination attempts, including a bomb that decapitated his chauffeur.
In 2003 Boris Berezovsky was granted political asylum in Britain on the grounds that his life would be in danger in Russia.
He was married twice and had six children – two with each of his wives and two with a long-term partner.
Boris Berezovsky ‘s wealth is thought to have considerably diminished in recent years, leaving him struggling to pay debts in the wake of costly court cases.
In 2011, Boris Berezovsky reportedly lost more than £100 million ($155 million) in a divorce settlement.
In 2012, Boris Berezovsky lost a £3 billion ($4.7 billion) damages claim against Chelsea Football Club owner Roman Abramovich.
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