Alexander Perepilichny, 44, collapsed and died in November 2012 while jogging near his house in southern England.
He had provided investigators with documents on an alleged scheme by Russian officials to defraud Hermitage Capital.
Two autopsies failed to establish a cause of death, though police said there was no evidence to suggest foul play.
At a hearing today in Woking, south of London, coroner Richard Travers said he was "determined" the long-delayed inquest would begin next month.