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 yesterday 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 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 his own 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.