(By Paul Virgo) Rome, July 30 - Judges at Italy's supreme court on Tuesday starting considering whether to uphold a four-year tax-fraud conviction against ex-premier Silvio Berlusconi. The supreme Court of Cassation's ruling is being hotly awaited because of the possible consequences it could have for Premier Enrico Letta's fragile left-right government, which needs the support of Berlusconi's centre-right People of Freedom (PdL) party to survive. If the Cassation upholds the verdict, the prison term and a five-year ban from holding public office for Berlusconi will become definitive. Some PdL lawmakers have threatened the party will pull its backing and sink the government unless the sentence is overturned, saying their leader is the victim of persecution by left-wing magistrates targeting him for political reasons. Berlusconi has said his legal problems and the government are separate issues. Letta said Monday he did not expect any "earthquakes" after the verdict. But some experts say the reassuring tone Berlusconi has taken and the way the three-time premier has reigned in PdL hawks has been dictated by lawyers' advice to prevent him irking the Cassation. The reality if the conviction is upheld, many suspect, could be different and the tension could be felt outside the courthouse on Tuesday, where groups of Berlusconi supports and opponents gathered. Letta's own centre-left Democratic Party (PD) could cause the government to collapse too if the conviction is confirmed as governing with an alliance partner led by a man with a definitive criminal conviction would be unpopular with rank-and-file members. Many in the party, the biggest group in parliament, were already unhappy about forming a coalition with their bitter rivals of two decades in the PdL in April to end two months of political deadlock after February's general election failed to produce a clear winner. 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. Prosecutors say this enabled Berlusconi to dodge taxes on around seven million euros in 2002 and 2003. "There is a thread that is given from the continuity of the system starting from the period of its invention in the 1980s," Prosecutor Antonello Mura told the Cassation Tuesday. Mura said the aim was to "inflate costs for tax benefits and produce payments for the creation of substantial capital abroad". Berlusconi says he had nothing to do with these dealings or authorising them as he was too occupied with political matters. He was premier at the time. Franco Coppi, a member of Berlusconi's defence team, said that the "crime does not exist", adding that he was aiming to have the conviction overturned. Berlusconi's lawyers have presented 50 objections to the conviction to the Cassation, which will rule on legal procedure and whether the lower appeals court properly justified its sentence. "I can certainly say that from observation of the ways in which the process was conducted, the rules have been respected and there is no contrast with the principles of just process," said Mura. 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 have to serve the year in prison if definitively convicted, but be given social work or house arrest as punishment. However, the five-year ban on holding office would kick in if, as usually happens, parliament ratifies it, in which case Berlusconi would have to step down as Senator, although this could take many months. At the weekend Berlusconi was quoted by a right-wing newspaper that he would go to jail, rather than asking for house arrest or try going into exile like disgraced former premier Bettino Craxi, his former friend, did after the Bribesville scandals of the early 1990s. Berlusconi's office subsequently said the media magnate had not given an interview to the newspaper, suggesting the comments were interpretations of a chat he had with the editor. The supreme court was not expected to hear the case until later this year, but earlier this month decided to accelerate proceedings to avert the risk of part of the conviction being timed out by the Statute of Limitations. The Cassation may uphold the sentence, overturn it, order a retrial or postpone a verdict. Franco Coppi, a member of Berlusconi's defence team, said Tuesday that the ruling was likely to be announced late Wednesday or on Thursday. 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 bribing a Senator to change sides to contribute to the fall of a previous centre-left government.