Silvio Berlusconi faces Ruby sex charge trial in April
Examining judge Cristina Di Censo said the process would start on 6 April, after prosecutors in Milan asked for an immediate trial.
Mr Berlusconi denies paying for sex with Karima El Mahroug when she was 17.
He also rejects claims that he abused his power by seeking her release when she was detained in another case.
He has called the accusations "groundless" and dismissed the case as a farce.
Mr Berlusconi does, however, acknowledge that he called the police while she was being held on suspicion of theft.
He said he was doing a favour for the then-Egyptian leader, Hosni Mubarak, because Mr Berlusconi was told the girl was Mr Mubarak's granddaughter.
Ms Mahroug, widely known as Ruby and now aged 18, has denied sleeping with the prime minister but has said she received 7,000 euros (£5,900, $9,400) from him as a gift after one of his parties.
On Sunday, hundreds of thousands of Italian women held nationwide protests against their embattled prime minister in more than 60 towns and cities across Italy and overseas.Payments to women?
Mr Berlusconi's fast-track trial in front of three female judges will start at a court in Milan at 0930 on Wednesday 6 April, the judge announced.
These are some of the most serious allegations Mr Berlusconi has faced during his long career, says the BBC's Duncan Kennedy in Rome. He faces three other court cases, but this is the first time he will face trial over his personal conduct.
The billionaire tycoon's lawyers have argued that the judge does not have the power to order the trial.
Although frequenting prostitutes is not a crime in Italy, having sex with one under the age of 18 is an offence that commands a prison sentence.
The prosecutors had submitted two sets of documents detailing the evidence against Mr Berlusconi. They allegedly included proof that payments were made by his aides to a "significant number" of young women, including Miss Mahroug.
Last month, Italy's Constitutional Court amended a law granting members of the government temporary immunity from prosecution. The court decided that individual judges should be allowed to decide whether ministers should be tried while in office.'Not a brothel'
Sunday's protests had a title - Se non ora, quando? (If not now, when?) - designed to express the frustration of those Italian women who are asking what it will take for Mr Berlusconi to resign, says our correspondent.
Some women carried banners reading "Italy is not a brothel", and said Mr Berlusconi had demeaned women with his recent sex scandals.
Rome's Piazza del Popolo - or People's Square - was crammed with tens of thousands of women and some men in an act of solidarity.
Despite all the recent negative publicity, Mr Berlusconi's opinion poll ratings are still at around 35%.
The billionaire prime minister also retains the support of his governing coalition allies, the Northern League, who do not want to see him quit, adds our correspondent.6*