Alexander Perepilichnyy, 44, collapsed in Weybridge in November 2012 after spending the night before his death with his mistress in Paris.
Coroner Nicholas Hilliard QC has been examining whether the married businessman could have been murdered with poison or if he died of natural causes.
The Old Bailey has heard a wealth of evidence that Mr Perepilichnyy had been blowing the whistle on alleged organised crime in Russia.
He had been helping UK-based campaigner Bill Browder's Hermitage Capital Investment £142 million money-laundering operation, the inquest was told.
Speaking ahead of the final Old Bailey hearing, Mr Browder said he had "no hunches".
He said: "I have no idea how this will go. We have been waiting for answers since 2012."
Perepilichnyy, a dad-of-two, was said to have appeared on a hit list in Moscow before his death.
He had taken out £3.5 million of life insurance and applied for another £5 million of policies amid concern to provide for his family, it was claimed.
A month before his death he had fought off a legal challenge by a debt recovery firm allegedly led by a prime suspect in the Alexander Litvinenko poison case, Dmitry Kovtun.
It was claimed an assassin may have wanted to kill him either to silence him or seek retribution and had the means to do it undetected.
Extensive tests have failed to identify any poison in Mr Perepilichnyy's body, although experts could not categorically rule out a toxin or even a nerve agent such as Novichok.
It was alleged an undetectable poison could have been used, or that the opportunity to identify it was lost in the days after his death.
Surrey Police have faced heavy criticism from Hermitage over its handling of the investigation.
Mr Browder, who describes himself as Vladimir Putin's "number one enemy", cited a "horrible lack of response" to a letter in 2012 alerting police to the organised crime connection.
He said: "We asked them to investigate his death as a potential murder and asked them to as quickly as possible secure the evidence and look for toxicology tests, based on the murder by poison of Alexander Litvinenko. The police didn't respond."
Not only were the contents of Mr Perepilichnyy's stomach thrown away, it also emerged that data on his computer had been lost.
It included alleged evidence of half-a-billion dollar transactions, threats and links to the money laundering case, the inquest was told.
On December 10 2012, Assistant Chief Constable Olivia Pinkney identified missed opportunities to alert senior management of issues surrounding the death, including a referral to Special Branch.
The coroner resisted calls for disclosure on whether Mr Perepilichnyy was in contact with British intelligence after a Government application for secrecy in the national interest.
Detective Superintendent Ian Pollard, of Surrey Police, has maintained there were no signs of "third party involvement or foul play".
The night before his death, Mr Perepilichnyy had been sick after a meal out at a fish restaurant in Paris with his ex-model lover Elmira Medynska, 28, the inquest heard.
While a post-mortem examination failed to identify a natural cause, it has been claimed the most likely explanation is sudden arrhythmic death syndrome.
Interested parties, including life insurers Legal and General, Hermitage, widow Tatiana Perepilichnaya and Surrey Police will reconvene at the Old Bailey on Wednesday to hear Mr Hilliard's findings.
In the absence of firm evidence that Mr Perepilichnyy was unlawfully killed, the coroner can reach an open conclusion or record death by natural causes.
LAST DAYS OF A RUSSIAN MULTI-MILLIONAIRE
November 9 2012: The married father spends the night with his ex-model girlfriend Elmira Medynska, 28, at the Buddha Bar in Paris.
November 10 2012: Mr Perepilichnyy, 44, returns from Paris where has been staying at the Hotel Bristol. His wife makes him sorrel soup and he collapses while out running near his Weybridge home.
November 14 2012: A first post-mortem examination is done. Nothing unusual is found in the stomach contents.
November 17 2012: Hermitage lawyers write to police urging them to investigate the death as a "potential murder", saying Mr Perepilichnyy was a whistleblower who had been co-operating with authorities and exposing Russian organised crime.
November 18 2012: Mr Perepilichnyy's immigration lawyer Roger Gherson calls Surrey police, claiming the Russian had been concerned for his safety in recent weeks and suggesting toxicology tests.
November 30: Second autopsy carried out and limited stomach contents are recovered.
June 2 2016: Pre-inquest review held by the Senior Surrey Coroner, Richard Travers.
November 2016: Home Secretary Amber Rudd wins High Court order preventing disclosure of sensitive material at the forthcoming inquest. The ruling says the position of the current Coroner, Richard Travers, was "untenable" because he did not have security clearance.
March 13 2017: Pre-inquest review held at Old Bailey by Coroner Nicholas Hilliard. Court hears claims Mr Perepilichnyy may have eaten poisoned sorrel soup on the day of his death. No evidence was found to back up the claim.
June 5 2017: Inquest gets under way at Old Bailey.
June 10 2017: Elmira Medynska says Mr Perepilichnyy appeared "very stressed" about his work and was "on another planet" when they went shopping to buy her a Prada bag and Louis Vuitton shoes. He was sick after a romantic dinner on their last night together, she says.
December 18 2018: Coroner Hilliard delivers his final conclusions in the long-running inquest at the Old Bailey.
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