Quebec's highest court has rejected Tony Accurso's appeal of a lower-court decision.
Accurso has argued that appearing before the Charbonneau Commission would jeopardize his right to a fair court hearing.
He faces criminal charges in several municipal corruption cases and is also charged with alleged tax fraud.
A Quebec Superior Court justice previously ruled that Accurso had not shown that his being subpoenaed to testify at the corruption probe would violate his rights to the point he should not have to appear.
Quebec Court of Appeal Justice Francois Doyon said in a judgment released today that commissioners have promised not to question Accurso about his pending criminal proceedings.
Accurso also went to the Supreme Court of Canada but the highest court in the land announced in April it would not hear his case.