Alexander Perepilichnyy, 44, collapsed and died in the road 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 (Â£147m) from the Russian state through tax fraud.
The full inquest had been due to get under way earlier, but was delayed because of the new toxicology information.
Tests 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.
A wealthy Russian businessman who died suddenly near his home in Surrey may have been poisoned, a pre-inquest hearing has been told.