The father-in-law of Pippa Middleton, whose sister is married to Prince William, is being investigated by French police over allegations he sexually assaulted an underage female, according to reports.

David Matthews, the father of James Matthews, was questioned by police Tuesday after his arrest at Orly Airport in Paris, Agence-France Presse reported. 

He was later released, The Telegraph reported.

The alleged assault happened between 1998 and 1999 and was reported to police last year. Matthews denies the "untrue and scandalous accusation."

Under French law, a formal investigation means there is "serious or consistent evidence" of probable involvement in a crime, according to The Telegraph. The investigation can be dropped without going to court.

Matthews, 74, is a millionaire and former racing driver who owns the Eden Roc resort hotel in St. Barts. He was at last year's marriage of his son to Pippa Middleton. Kate Middleton was maid of honor and Princes William and Harry also also attended. Princess Charlotte and Prince George were served as flower girl and page boy. 

A spokesman issued a statement Friday. “David Matthews categorically denies the allegation and unequivocally contests the untrue and scandalous accusation,” it said.

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