Specifically, Amorth cites recent sexual abuse and pedophilia scandals as well as what he deems a cover-up in the shooting deaths of two of the Vatican's Swiss Guards and one of the guard's wives as proof that the Catholic Church's most famous site is less than pure.
"When one speaks of 'the smoke of Satan' in the holy rooms, it is all true -- including these latest stories of violence and pedophilia," Amorth was cited as saying by The Times of London. The smoke of Satan references a phrase coined by Pope Paul VI.
The Vatican, according to Amorth, was also home to "cardinals who do not believe in Jesus and bishops who are linked to the demon."
Many of Amorth's claims are made in a new autobiography, titled "Memoirs of an Exorcist."
Amorth, who is the founder and president of honor of the Association of Exorcists, became an official exorcist in the Catholic Church in 1986, The Times reports. But Amorth's diagnosis may not mean that the church is preparing a Vatican exorcism any time soon.
Saying that Amorth had "gone well beyond the evidence" for Satanic possession of the Vatican, Rome-based exorcist the Rev. José Antonio Fortea Cucurull told The Times, "Cardinals might be better or worse, but all have upright intentions and seek the glory of God."
In an interview in 2002, Amorth was asked what defines an exorcism.
"Exorcism is a public prayer of the church done with the authority of the church," Amorth replied. "Because it is done by a priest name by the bishop; it is a prayer for the liberation from the demon, from the evil influence of the demon or of the evil provoked by the demon."
Popular conceptions of the practice are largely based on the 1973 film "The Exorcist," which Amorth called "exaggerated," but also praised for presenting an accurate picture of the possessed.
In the past, Amorth has attributed events ranging from fires in Sicily to the business woes of the airline Alitalia to the workings of Satan.