A civil marriage suffices anywhere, as evidenced by the 1983 wedding of Princess Caroline of Monaco and her second husband, Stefano Casiraghi -- which happened years before her first marriage (to Philippe Junot) was finally annulled by the Vatican. By then, it was too late to validate the couple's marriage in the Catholic Church, since he had died two years previously. But it didn't matter: all three of their children were born in line to the Monegasque throne, since the parents were legally married (Prince Rainier III having given his consent).
But for some reason Princess Caroline petitioned the Holy See to grant a canonical legitimation of her three children by the late Stefano Casiraghi. This is an exceptionally rare procedure.
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