* Former senator says took 3-mln-euro bribe from Berlusconi
* Senator's defection to Berlusconi bloc destabilised gov't
Former Italian Prime Minister Silvio Berlusconi is under investigation on suspicion of bribing a senator to change sides in parliament, deepening the legal troubles of one of the key players in the country's post-election deadlock.
Berlusconi's lawyer Niccolo Ghedini said the accusation "was without foundation".
The allegations were detailed in a document from prosecutors posted on Thursday on the website of Italy's parliament, which must approve the court's request to search a Berlusconi security deposit box and access his phone records.
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 one with a majority in parliament.
According to the document, former Senator Sergio De Gregorio told prosecutors Berlusconi paid him 3 million euro ($3.92 million) to switch parties in 2006, a move which destabilised the centre-left government and contributed to its eventual collapse.
"I told Berlusconi what I wanted... that the party should give me, either the party or he personally, should pay me 3 million," De Gregorio is quoted telling investigators, adding that he needed the money to get out of financial difficulties.
No comment was immediately available from De Gregorio's lawyer.
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 Berlusconi's PDL, in 2008.
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".
In a statement, PDL 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 response, the head of the National Magistrates Association said: "We firmly reject the accusations, repeated periodically, that justice is used for political means."
Berlusconi often states that the cases against him are cooked up by "leftist judges" to damage him politically.
Berlusconi is 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.