Surrey Police said Alexander Perepilichnyy's death was not suspicious
Alexander Perepilichnyy, 44, collapsed and died in the road near his home in Weybridge, Surrey, on the evening of 10 November 2012.
Surrey Police investigated his death, but decided it was not suspicious.
However, new toxicology tests have shown that a rare poison may have been used to kill him.
Mr Perepilichnyy had told colleagues in London he had received death threats, reports BBC world affairs correspondent Richard Galpin.
He had handed over sensitive documents to a businessman in London and to prosecutors in Switzerland, our correspondent adds.
These implicated Russian officials and mafia members in the theft of $240m (£147m) from the Russian state through tax fraud.
Russian officials and Mafia members are one and the same, aren't they? This is almost certainly in reference to the same tax fraud that was perpetrated on the Russian people by the government after they kicked Bill Browder out of Russia, stole his businesses, and murdered his lawyer, Sergie Magnitsky.
See Who's Bill Browder and Why Does He Want To Bring Down Putin
The full inquest had been due to get under way earlier, but was delayed because of the new toxicology information.
Tests had showed a potential marker for a rare poison called Gelsemium elegans. Further tests will be carried out to establish if the poison was in his body.