The former art dealer, who is said to "obsess" Diana, died at the age of 73 at his home in France, the Daily Mail reports.
The married father of three was a close friend of Charles.
He is said to have started an affair with Charles's wife Princess Diana in 1992 following the death of her father Earl Spencer.
Princess Diana allegedly confessed to her confidant Lady Bowker that she "dreamed of living in Italy with pretty Hoare".
A former bodyguard told how he saw Mr. Hoare "half naked behind a sunken laurel tree in a corridor in Kensington Palace smoking a cigar".
His affair with Diana ended when his French-born wife Diane de Waldner – 16 years older than Diana – threatened divorce.
Mr. Hoare was a friend of Charle and he and his wife, with whom he stayed married, had been guests at Windsor Castle.
But when his wife threatened to divorce in 1993, he broke off the affair.
This is said to have prompted Diana to bombard his house with phone calls.
Mr. Hoare is said to have contacted the police about the phone calls.
In an interview with Panorama in 1995, Diana admitted that she called him but said, "Over a period of six to nine months, a few times, but certainly not in an obsessive way."
Old Etonian never spoke of his affair with the Princess of Wales.
Mr. Hoare was a student of art history at the Sorbonne before joining Christie's in 1967.
He later became an Islamic art specialist and is said to have sold Sheikh Saud Al Thani of Qatar art worth more than £ 20 million