(By Paul Virgo) Rome, July 29 - Premier Enrico Letta said Monday he was not worried about his left-right government collapsing ahead of the supreme Court of Cassation's ruling on a four-year prison sentence against ex-premier Silvio Berlusconi. If the Cassation, which is set to hold a hearing on the tax-fraud case Tuesday, upholds the verdict, the prison term and a five-year ban from holding public office will become definitive. Some lawmakers for Berlusconi's centre-right People of Freedom (PdL) party have threatened it will pull its backing and sink the government unless the sentence is overturned, saying their leader is the victim a persecution by left-wing magistrates targeting him for political reasons. There is also speculation Letta's own centre-left Democratic Party (PD) could pull out as many elements within would be unhappy about governing with an alliance partner led by a figure with a definitive criminal conviction to his name. Three-time premier Berlusconi, meanwhile, has said his legal problems and the government are separate issues. And Letta also seems confident his fragile administration, which took power in April after two months of deadlock following February's inconclusive general election, will survive no matter what the Cassation decides. "I don't think there will be the earthquakes evoked by those who, evidently, want earthquakes," Letta, whose government is also threatened by policy differences between the PD and the PdL, said during a visit to Greece. "I'm convinced that the situation is much more stable than it is being presented as... I'm absolutely relaxed and serene. "I've planned the work for next year and I'm convinced that stability will be confirmed". The PdL managed to halt parliamentary business for a day after the Cassation said it would hear the fraud case on July 30, rather than later this year as had been expected. The court said this was necessary to stop part of the accusations against Berlusconi being timed out by the statute of limitations next month. The four-year conviction regards a system of inflated film-rights purchases at Berlusconi's Mediaset media empire and the use of offshore companies to create slush funds and dodge taxes. Berlusconi says he had nothing to do with these dealings or authorising them as he was too occupied with political matters. Because of a 2006 amnesty law, three of the four years of the sentence will not be effective, if the sentence is confirmed. As he is over 70, he would probably not serve the year in prison if definitively convicted, but be given social work or house arrest as punishment. However the five-year ban would kick in if, as usually happens, parliament ratifies it, in which case Berlusconi would have to step down as Senator. The Cassation may uphold the sentence, overturn it, order a retrial or delay proceedings, if this is requested by Berlusconi's defence team. Berlusconi has faced many criminal cases since becoming a politician, but he has never received a definitive conviction at the end of the appeals process. Several were timed out. Berlusconi is also appealing against a seven-year sentence and a life ban from office for paying an underage prostitute nicknamed Ruby for sex and a one-year term for involvement in the publication of a wiretap that hurt a political rival. He may also face trial for allegedly buying Senators to bring down a previous centre-left government.