The case in a Milan court focuses on publication of a conversation from a wiretap in Paolo Berlusconi's conservative newspaper, Il Giornale, the Italian news agency ANSA reported.
The 2005 conversation between Piero Fassino, who had headed the opposition Democratic Left Party, and Giovanni Consorte, the former chairman of Unipol, an association of insurers that has historically been connected to the DS, Italy's former Communist Party, was recorded as Unipol came close to taking over one of Italy's leading banks, Banca Nazionale del Lavoro.
"We have a bank!" Fassino was recorded saying.
Fassino, now the mayor of Turin, had faced widespread criticism for the remark, particularly among rank-and-file members of the DS, now a larger center-left group, the Democratic Party, ANSA said.
Berlusconi had told a Milan court before his indictment he hadn't heard the wiretap. "Otherwise, I would have remembered," he said.
Paolo Berlusconi was indicted in June. Silvio Berlusconi's case was sent to trial in February.
Two other people involved have been convicted of involvement in the wiretapping.