Both Berlusconi, 77, and el-Mahroug, 21, have denied having had sex. Berlusconi firmly denies the charges, including one that he used his influence to force a public official to cover up the under-age prostitution charge, and claims that the case is part of a judicial plot to sideline him politically.
Berlusconi's defense lawyers called the reasoning "surreal" and expressed confidence the verdict would be overturned on appeal.
The filing of the court's reasoning paves the way for Berlusconi's defense to appeal the verdict and sentence. It also comes just a week before the Senate is expected to vote on whether to expel him on the basis of a tax-fraud conviction that was confirmed this summer by Italy's highest court.
Berlusconi, who has no role in government, has been weakened by the judicial woes as well as his failed attempt to bring down the government last month. In a bid to recoup his past glory, Berlusconi has relaunched the Forza Italia party that catapulted him to political power two decades ago, some of his closest allies abandoned him to start their own center-right party. The breakaway group said Berlusconi's judicial fate cannot decide whether the struggling government survives.
Neither Berlusconi nor el-Mahroug testified in the under-age prostitution trial.
El-Mahroug, however, did testify in the trial of three Berlusconi aides who were convicted in July of procuring prostitutes for the parties. She told that court that Berlusconi's disco featured aspiring showgirls dressed as sexy nuns and nurses performing striptease acts, and that one woman even dressed up as President Barack Obama. She denied she had ever acted as a prostitute, and any claims she had made to others about having sex with Berlusconi or receiving millions of euros from him were lies intended to inflate her own importance.
The court concluded Ruby had prostituted herself in other situations and was a participant in the sexually charged Bunga Bunga parties, at one point telling another young woman "Now I dance, then strip, then have sex."
The court said the details of the sexual encounter with Berlusconi are "irrelevant," rather it was sufficient that she was "trading her body in any way for remuneration."
El-Mahroug has not been charged with any offense.