Speaker Piero Grasso said said it was his "moral duty" to declare the Senate a civil party despite an earlier recommendation by a parliamentary
committee for the upper house to stay out of the media magnate's latest legal troubles.
Embattled Berlusconi was ousted from parliament and stripped of legal protection in November after he was found guilty of tax fraud.
He is now accused of bribing of a left-wing senator to defect to his centre-right party in 2006 to bring down premier Romani Prodi's government.
The trial against him is expected to start in Naples next Tuesday.