By Barry Moody
ROME (Reuters) - Italy's supreme court convenes on Tuesday to rule whether Silvio Berlusconi should be jailed and banned from public office for tax fraud, a verdict that could endanger Italy's shaky coalition government.
A ruling against the former prime minister would be his first definitive conviction and signal the end of an era in which he has dominated Italian politics for two decades through his media power and political skill.
It could also plunge the government - an uneasy coalition of Enrico Letta's center-left Democratic Party (PD) and Berlusconi's center-right People of Freedom (PDL) - into crisis and bring renewed uncertainty to the euro zone third's largest economy that could reverberate across the bloc.
The five judges of the Cassation Court - Italy's highest -will hear a final appeal by Berlusconi against a 1-year jail sentence and 5-year ban from office handed down by lower courts for the fraudulent purchase of broadcasting rights by his Mediaset media empire.
The court may take up to three days to deliver its ruling.
Berlusconi, 76, accuses leftwing magistrates of trying to bring him down in more than two dozen court cases since he stormed to power for the first time in 1994.
Although they are waiting for a signal from Berlusconi, PDL hawks have called for everything from a mass resignation of its government ministers to blocking Italy's motorways if the court rules against him.
Fabrizio Cicchitto, a senior PDL parliamentarian, said the media magnate had faced 30 trials. "If this is not a political use of justice what is?"
The departure of Berlusconi from parliament if he is convicted would also raise major questions about the future of his party, which depends on his charisma and wealth.
But a greater risk to the government could come from Letta's faction-ridden PD, many of whose members are already deeply unhappy with being in a coalition with their old enemy, and may refuse to continue if he is found guilty.
However, both President Giorgio Napolitano, who dragged the parties into a coalition in April after a two-month crisis that followed inconclusive elections, and Letta himself are adamant that Italy cannot afford more instability as it struggles to climb out of its worst postwar recession.
Both of the major parties may also be reluctant to precipitate an election whose result could be even more chaotic than the February vote and boost support for the populist 5-Star Movement of comedian Beppe Grillo.
Berlusconi has for months kept his hawks on a tight leash, saying the government must continue.
However, political sources say this stance was dictated by Berlusconi's lawyers, who wanted to avoid upsetting the supreme court judges. The mercurial magnate's reaction if he is found guilty is uncertain.
Berlusconi's lawyers have filed 50 objections to the supreme court, which will rule only on legal procedure and whether the previous appeal court properly justified its sentence.
The court has three choices: convict Berlusconi, acquit him or send the case back to the appeal court because of legal errors. It could also postpone a decision, probably until September - a move advocated by moderate politicians who want to avoid a summer crisis.
Even if Berlusconi is found guilty, his ban from public office depends on a vote by his peers in the Senate which could take weeks or months.
The scandal-plagued mogul is also appealing against a seven- year jail term handed down in June for abuse of office and paying for sex with Moroccan-born nightclub dancer Karima El Mahroug alias "Ruby the Heartstealer" when she was underage.
(Additional reporting by Catherine Hornby and James Mackenzie; Editing by Robin Pomeroy)