In an interview on Italy's Annozero, a Rai 2 investigative chat show, self- confessed escort Nadia Macri revealed the alleged details of her encounters with Berlusconi, then pulled out her cell phone to reveal "Cav. Presidente Silvio Berlusconi," among her telephone contacts.
Revealing the number to cameras, the interviewer hastily covered the last three numbers with his finger, an image which has now made the front page of the Italian daily Il Giornale.
Recently, the last three digits of the premier's number had been published on Indymedia, according to Italian media reports, making it possible to decifer the entire number.
Macri was due to be questioned by Milan prosecutors on Friday over a series of wiretap transcripts referring to payments she alleged received from Berlusconi's financial administrator.
She made headlines last November when she came forward and claimed to have had sexual relations with the premier on two occasions and to have received 10,000 euros from him.
Macri also claimed to have had relations with public administration minister Renato Brunetta and with the mayor of Parma, Pietro Vignali.
Macri is just one of a number of young women named in a widening prostitution probe centred on a teenage Moroccan nightclub dancer who alleged had sex with Berusoconi when she was aged less than 18.
Prostitution is not a crime in Italy, but using an underage prostitute is.