Revelations that the Vatican encouraged the castration of choir boys in the name of art for hundreds of years have prompted calls for a papal apology.
Human rights groups, historians and Italian commentators said the Pope, a singer himself, should ask forgiveness for his predecessors' role in the mutilation of castrati singers.
New research suggests that the employment of castrati was tolerated by the Vatican as late as 1959, long after other states had banned it as barbaric.
From the 16th century onwards generations of Italian boys were castrated in the hope that their voices, prevented from breaking, would combine a child's high register with the vocal power of a man.
Their ability to sing beyond normal human limits enraptured opera-goers, emperors and popes, who commissioned a choir of castrati to perform in the Sistine chapel. An edict by St Paul prevented women singing in church.
Successful castrati such as Farinelli - the subject of Gérard Corbiau's 1994 film - became Europe-wide superstars, feted by composers such as Handel, but most failed to make the grade and were cast aside, devastated and useless even as circus freaks.
According to Angels Against their Will, a new book by the German historian Hubert Ortkemper, the castrato Alessandro Moreschi performed in the Sistine chapel until 1913. Other historians suspect that Domenico Mancini, another private pontifical singer who performed from 1939 to 1959, was a castrato, too.
Officially the Vatican always condemned the practice, which is thought to have started around 1500, and punished castrators with excommunication. In 1902 it issued a decree banning castrati from the Sistine chapel.
But such was the beauty and power of their singing that successive popes sponsored the phenomenon by employing them on the pretext that they were accidentally castrated, for example by falling from a horse or by an animal bite.
Italy's leading newspaper, Corriere della Sera, said the Pope, whose CD recordings have sold millions, should follow up his admission of church wrongs against Jews, Muslims and scientists by expressing sorrow for the castrati.
"Despite the willingness to address just about any issue, the current pope has yet to confront an unresolved problem of musical history. Why doesn't he suggest prayers and remorse for the church's past connivance with the practice of castrating males?"
Human rights activists and academics endorsed the call. Amnesty International said the value of recognising past wrongs in an apology should not be underestimated.
"Many of those afflicted by ongoing human rights abuses - including genital mutilations of women and rape as torture - desperately desire official recognition of the terrible wrongs done to them. An apology from those involved may be the hardest thing of all to achieve, and the most valued."
Nicholas Davidson, an Oxford University expert on papal history, said: "If the Pope was going to be consistent, and if there was evidence that church officials operated in an improper way, then an apology should be made."
The promise of a lucrative career persuaded many poor Italian parents to castrate sons with musical talent, despite the fact that the operation often produced gigantism and obese bottoms and legs.
No records were kept, but historians believe many operations to remove testicles - achieved by slitting the groin and severing the spermatic chord - were botched, leaving boys in agony and in danger of infection. The lucky ones survived and were good enough for years of intensive training and cossetting at musical academies.
Pope Sisto V, aware that the public craved the "voice of angels", sanctioned their presence in the Vatican by a papal bull in 1589.
Audiences fainted and wept during performances and groupies wore medallions of their favourites, but in the 18th century the practice was gradually acknowledged to be grotesque.