Berlusconi's relationship with Ruby became public last year, when it emerged that the prime minister had intervened when police arrested her for theft. Berlusconi is accused of calling the police in March and sending Minetti to intervene. Minetti is also under investigation.
Berlusconi reportedly told the police Ruby was Mubarak's niece. The Egyptian embassy in Rome denied the relationship.
The premier has admitted to calling the police on Ruby's behalf, but claims he did so because she was an underprivileged girl in trouble.
Berlusconi has denied the accusations since they emerged late last year. He has called the investigation and indictment against him "groundless" and a "farce."