Elisabetta Canalis, 31, has been dating the actor for a year and there are rumours that they are due to get married this weekend at his lakeside villa.
But according to Italian newspapers Corriere Della Sera and La Repubblica, Ms Canalis features in an investigation being conducted into cocaine use at Milan nightspot The Club.
The allegations have been made by Karima Menad, 26, said to be a wannabe model from Paris who turned to prostitution after failing to find fame in Milan.
Reports in Italy say that Menad gave her testimony in 2008 and that it had emerged after prosecutor Frank Di Maio used it as part of his official request to have the owners of The Club held in custody while the probe continued.
Besides The Club, another famous Milan nightspot, Hollywood, is also said to be under investigation by prosecutors after clients were secretly filmed by police allegedly taking drugs in the toilets.
Both are favoured by the Italian jet set as well as passing high-profile VIPs including David Beckham.
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