Prime Minister Elio Di Rupo made the announcement today, and the country's media said Mortier died the day before at his home in Brussels after a protracted battle with pancreatic cancer.
Di Rupo called Mortier a "visionary and generous personality," praise echoed by French President Francois Hollande, who said, "He never stopped fighting, until the end of his strength, for culture in Europe."
Teatro Real, where Mortier was serving as artistic adviser, said in a statement: "He contributed to promoting Spain's operatic and cultural landscape and placing the Teatro Real among the world's leading international opera houses."
The artist became the director of Belgium's National Operation, known as La Monnaie, in 1981, steering it away from "bourgeois" entertainment and to international recognition and acclaim.
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At a time when much of opera was still ensconced in the regalia of "old" art, Mortier relied on people such as director Peter Sellars and composer Philip Glass to push the artistic envelope, sometimes to a breaking point.