Rome, August 1 - Five judges at Italy's supreme court huddled on Thursday to decide whether to uphold a four-year tax-fraud conviction against ex-premier Silvio Berlusconi. The decision is expected to be announced at around 17:00 Italian time. If the sentence, which comes with a ban from holding public office, is confirmed, it will be the first time Berlusconi has received a definitive conviction after dozens of cases against the media magnate since he embarked on a political career two decades ago. The supreme Court of Cassation's ruling is being hotly awaited, in part 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. Three of the four years of the sentence would not be effective if the conviction is upheld because of a 2006 amnesty. As Berlusconi is over 70, he would probably not have to serve the remaining year in prison, but be given social work or house arrest as punishment. Berlusconi is also appealing against a five-year ban on holding office, which State prosecutors have requested be reduced to three years. The ban 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 months. 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. There have also been threats that they would quit en masse and that eventual protests would include road blocks on motorways. Berlusconi has said his legal problems and the government are separate issues and 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. 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. His defence team have also argued that any wrongdoing is an accounting offence punishable by a fine and not fraud. 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. Berlusconi has faced many criminal cases since becoming a politician, but he has never received a definitive conviction at the end of Italy's two-tier appeals process. Several cases 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 in separate cases. He may also face trial for allegedly bribing a Senator to change sides to contribute to the fall of a previous centre-left government.