ROME, Feb 5 (Reuters) - Italy's centre-right launched a fierce attack on the speaker of the upper house on Wednesday over his decision to have the Senate join a criminal case as an injured party against former Premier Silvio Berlusconi.
The attack adds to an already poisonous atmosphere in parliament just as the parties are about to begin parliamentary debate to seek agreement on a new electoral law aimed at ending Italy's chronic political instability.
Senate speaker Piero Grasso decided the Senate should declare itself a civil party in the case in which Berlusconi is accused of bribing centre-left senators to change sides to bring down the government of Romano Prodi between 2006 and 2008.
Grasso, a former anti-mafia magistrate, said it was his "moral duty" to overrule a parliamentary committee recommendation for the Senate to remain outside the case, which opens in Naples on Feb. 11.
Maurizio Gasparri, deputy speaker to Grasso and the Senate leader of Berlusconi's Forza Italia party, called Grasso's decision "disgraceful," and appealed to President Giorgio Napolitano to intervene.
Maurizio Sacconi, Senate leader of the New Centre-Right, which broke away from Forza Italia last year but remains a likely ally in case of elections, called it "an ugly day for our institutions."
Berlusconi is already barred from parliament following a separate tax fraud conviction.
The Senate leader of the the centre-left Democratic Party, the mainstay of the cross-party coalition backing the government of Prime Minister Enrico Letta, defended Grasso and called the centre-right's accusations "absurd."
The incident comes after days of escalating tensions in parliament with the anti-establishment 5-Star Movement at the centre of a row with the speaker of the Chamber of Deputies, Laura Boldrini.
On Wednesday a group of 5-Star members, including several parliamentarians, announced they were sueing Boldrini for having called their party supporters "potential rapists." (Reporting By Gavin Jones; editing by Ralph Boulton)