ROME, Feb 28 (Reuters) - Former Italian prime minister Silvio Berlusconi, challenging for a share of power after this week's election, has been placed under investigation on suspicion of bribing a senator to change sides in parliament in 2006, sources in the investigation said on Thursday.
Two of Berlusconi's lawyers, Niccolo Ghedini and Piero Longo, said they had not yet received any official notification of the investigation from Naples magistrates and would not comment until they had.
The fresh accusations come as parties including Berlusconi's centre-right People of Freedom (PDL) struggle to form a government after this week's inconclusive election, which left no party with a majority in parliament.
The prosecutors are investigating allegations that a senator, Sergio De Gregorio, was paid 3 million euros ($3.9 million) to leave his party and join the PDL, the sources told Italian news agencies.
De Gregorio's move helped to trigger the fall of the last centre-left government, headed by Romano Prodi.
No comment was immediately available from De Gregorio's lawyer.
The PDL did not confirm the investigation but a statement from party secretary Angelino Alfano said "the aggression by magistrates against Silvio Berlusconi is beginning again". He said the party would organise a demonstration "to defend the sovereignty of the People of Freedom and Italian democracy".
In a separate case, prosecutors in Reggio Emilia have opened an investigation into Berlusconi's campaign pledge to return property taxes paid last year if the centre-right won the election.
The case was opened after two formal complaints filed by citizens, alleging that the offer constituted vote buying. PDL official Deborah Bergamini said in a statement that the investigation was "an illogical action aimed at intimidating anyone whom magistrates do not like".
Renewed scandal surrounding Berlusconi could reduce the willingness of the other major political groups, the centre-left and the 5-Star Movement, to form a government with the former prime minister.
The new allegations emerged during a separate corruption probe against De Gregorio, who left the Italy of Values party in September 2006 and was re-elected as a senator, this time for the PDL, in 2008.
Berlusconi is currently on trial for tax fraud in a case centring on the purchase of broadcasting rights by his Mediaset group, for making public the taped contents of a confidential phone call, and for paying for sex with an underage girl.
He has faced up to 30 prosecutions for fraud and corruption over his career, but has never been definitively convicted. ($1 = 0.7628 euros)