Italy's top bishop has issued scathing criticism of Premier Silvio Berlusconi for his alleged role in a sex scandal, insisting that public officials must control themselves.

Cardinal Angelo Bagnasco, the head of the Italian bishops' conference, warned of the damage to the country and its reputation and said Italians were fed up with the scandal and its domination of the political scene.

He added that the matter should be resolved quickly.

Prosecutors have placed Berlusconi and three associates under investigation, alleging he paid for sex with a 17-year-old girl nicknamed Ruby and used his office to cover it up.

Berlusconi has denied the allegations and accused prosecutors of a politically motivated witch hunt.

Ruby, who is now 18, has denied she had sex with the premier, but has said he gave her $9,400 to help her out financially.

Wiretapped conversations of participants at parties - printed in virtually every Italian newspaper - have described Berlusconi's villa as a brothel with topless girls.

Berlusconi, meanwhile, has insisted the dinner parties were perfectly correct and denied ever paying for sex.

Despite his aggressive defence, the conservative Berlusconi has come under increasing criticism from the Catholic Church, with the Vatican Number two, Cardinal Tarcisio Bertone, referring directly to the probe last week by calling for a more "robust morality" and legality among public officials.

Pope Benedict XVI issued a similar call a day later, though he didn't mention Berlusconi by name.

Paying for sex with a prostitute is not a crime in Italy, unless the prostitute is younger than 18.