Justice France Charbonneau, who heads the provincial probe into organized crime involvement in the construction industry, has rejected a bid by Raynald Desjardins to get out of appearing.
Charbonneau said in her ruling on Thursday that Desjardins has information pertinent to the inquiry, noting he has links to a company tied to organized crime and a labour union boss.
Desjardins' lawyer had argued that his client is already awaiting trial for the first-degree murder of a former boss with the Bonanno crime family.
Marc Labelle said it would be impossible for Desjardins to get an impartial jury if he appeared at the corruption inquiry.
But in her ruling the inquiry chair says it is difficult to argue his rights will be violated because no date has been set for his appearance at the probe.
News reports have described Desjardins as among the most senior figures in Canada's Italian Mafia and an instrumental player in the recent civil war in the underworld.
He was arrested and charged with killing a reported aspirant in the race to supplant the Rizzuto family. More recently, Desjardins' brother-in-law Joe Di Maulo was shot to death in his driveway.