Berlusconi and co-defendant Valter Lavitola are accused of corruption over an alleged pay-off to former senator Sergio De Gregorio to switch sides several years ago from the rival centre-left to Berlusconi's centre-right bloc.
The switch eventually led to the collapse of a former government. Mr Lavitola's lawyer Guido Iaccarino said a judge in Naples yesterday decided a trial was needed "to determine the truth".
In a plea-bargain deal, De Gregorio was sentenced to 20 months in prison for corruption. The defection of the Naples senator to Berlusconi's People of Freedom party weakened the government of Romano Prodi which collapsed in 2008 and triggered elections that returned Berlusconi to power.
Berlusconi, 77, has denied corruption. He has already been found guilty in two other trials of tax fraud and having sex with an under-age prostitute.
On Saturday a court in Milan banned him from holding public office for two years, following his conviction in August for tax fraud.
The ban has still to be voted on in parliament.