Italy's former prime minister Silvio Berlusconi is facing fresh criminal charges for allegedly bribing a political opponent to help bring down the left-leaning government of Romano Prodi in 2006.

Naples prosecutors have opened an investigation into allegations of a €3m (£2.6m) bribe they believe Berlusconi paid to former senator Sergio De Gregorio to drop support for Prodi's shaky government and help bring it down.

Berlusconi has been accused of corruption and illicit funding in collusion with De Gregorio and suspected wheeler-dealer Walter Lavitola.

In 2006 De Gregroio was elected to the anti-corruption and anti-Berlusconi party Italy of Values (IDV). It was led by former prosecutor Antonio Di Pietro, who had a prominent role in exposing Italy's biggest bribery scandal, "Tangentopoli", in the 1990s. IDV joined Prodi in a ruling alliance.

Shortly after the elections, however, De Gregorio quit IDV and founded his own party - Italians in the World - which aligned itself with Berlusconi's People of Freedom party (PDL). De Gregorio was subsequently appointed president of the senate defence committee.

The switch in allegiance was a hard blow for Prodi's government which had been clinging on to power with a three-seat majority in the senate. It collapsed in May 2008.

Di Pietro said De Gregorio had "sold his soul."

Later, De Gregorio admitted on TV that Berlusconi had pledged €500,000 in funds for his electoral campaign. Naples prosecutors Vincenzo Piscitelli and Henry Woodcock believe, however, that he was actually remunerated to the tune of a €3m bribe, negotiated by Lavitola.

"Once elected [De Gregorio] joined the centre-right thanks to Lavitola's mediation with Berlusconi," Lavitola's accountant told prosecutors. "The agreement was handsomely rewarded."

Lavitola is already facing a series of charges in connection with corrupt Panamanian politicians and for having extorted money from Berlusconi over the bunga-bunga affair - a claim Berlusconi denies.

PDL deputy-chairman for the lower chamber, Maurizio Lupi, accused the prosecutors of "using justice for political goals" to undermine Berlusconi's political comeback.

"The opening of a new prosecution two days after the elections would be incredible and trigger outrage in any other country," Lupi said.

Sergio De Gregorio
Italy’s former senator Sergio De Gregorio (Facebook)