Monaco's Princesses Caroline and Stephanie reportedly boycotted the Monte Carlo Red Cross ball because of the entertainment.
The royals missed the annual gala - which was organised by the princesses' brother Prince Albert and his wife Princess Charlene - at the Sporting Club Salle des Etoiles last week, reportedly because they disagreed with their sister-in-law's insistence on booking her jazz singer friend Diana Krall to perform.
And while the hosts - who are expecting twins - applauded the performance by the piano virtuoso, it was said to have fallen flat with many of the other French and Italian guests in attendance.
One source told the New York Post newspaper's Page Six column: "It was terrible, a disaster. Very, very boring."
As well as the princesses, Caroline's daughter Charlotte Casiraghi also skipped the gala, as did other regular guests including Karl Lagerfeld and Lynn Wyatt.
Charlene and Albert will welcome their children into the world in December and the couple, who got married in 2011, previously revealed they were filled with "immense joy" at the thought of expanding their family.
A joint statement from the duo read: "Prince Albert and Princess Charlene have the immense joy to announce they are expecting a happy event. The birth is due at the end of the year."
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