A Milan court on Monday rejected a defence request to move a sex trial against Prime Minister Silvio Berlusconi to a special tribunal for ministers in Rome.
Berlusconi's lawyers say the Milan judges are biased against him. His critics say he is trying to escape ordinary justice and would get a more favourable hearing in the tribunal. The final decision is due to be made by Italy's constitutional court.
Moving the case would force prosecutors to start again from scratch, voiding a large part of their work on the case so far.
Berlusconi, 74, denies charges that he paid for sex with Moroccan teenager Karima El Mahroug, a nightclub dancer known by her stage name Ruby. He went on trial in April on charges he gave her cash and jewels in exchange for sex when she was 17. He is also accused of abusing his powers to cover up the affair.
The media magnate admits a fondness for young women but denies having sex with Ruby and dismisses accusations of wild sex parties at his residence.
The constitutional court ruled earlier this month that it could consider an appeal by the centre-right controlled lower house of parliament, arguing that the ministerial tribunal in Rome should hear charges against Berlusconi of abuse of power, which are part of the prostitution case.
The Rome court is expected to decide by the end of the year.
Berlusconi did not attend the Milan hearing on Monday as he was due to meet the head of state in Rome to discuss Italy's next steps after passage of an austerity budget.
Another hearing in a separate Berlusconi trial on corruption charges is also taking place in Milan.