Italy's constitutional court yesterday began deliberating whether to strip Silvio Berlusconi of his immunity from prosecution against a backdrop of alleged subversive plots and talk of a snap election.

Already on the defensive because of a lurid sex and drugs scandal, Italy's prime minister faces going back on trial in two cases if the act passed last year to shield him from the law is ruled unconstitutional.

In his address to the court, Berlusconi's lawyer Gaetano Pecorella said that, as a result of changes to the electoral law, the prime minister could no longer be regarded in the same way as other politicians. Pecorella told the judges that Berlusconi was "no longer 'first among equals', but ought to be considered 'first above equals'."

The court had been expected to rule today, but may have to delay its decision if there are deep divisions of opinion between the judges. They can endorse or overturn the immunity law altogether, or veto parts of it – which would allow the government to come up with an amended version.

For more than 15 years the media tycoon turned conservative politician has been proclaiming that he is a victim of systematic persecution by leftwing judges and prosecutors. Many, if not most, Italians now believe him.

One of Berlusconi's most senior colleagues, defence minister Ignazio La Russa, declared yesterday that if the law were thrown out it would be "a political decision, more than a juridical one".

Some of Berlusconi's associates were pressing for street demonstrations if the court decided against him. But influential allies in the Northern League said the prime minister would do better to go back to the country for a renewed mandate.

In a statement on Monday the leaders of Berlusconi's party in parliament issued a statement claiming a "subversive plan" was being enacted to "challenge the democratic will of the Italian people".

The 15 members of Italy's most august tribunal began considering the immunity act the day after a judge in Milan again drew attention to Berlusconi's history of problems with the law. Judge Raimondo Mesiano ruled that Italy's prime minister had been "jointly responsible" for the bribing of a judge in a trial that was crucial to the expansion of his business empire.

Mesiano was giving the reasons behind a decision on Saturday to award €750m (£695m) damages against Fininvest, the financial holding company at the apex of the Berlusconi group. Fininvest was told to pay damages to the rival CIR group as compensation for having bribed a judge to ensure it won a battle for control of the publishing group Mondadori.

Berlusconi's former lawyer, Cesare Previti, was convicted of bribing the judge two years ago. CIR owns La Repubblica, the centre-left daily which has led a campaign to get Berlusconi to answer a string of allegations about his private life.

The law under discussion yesterday grants immunity from prosecution to Italy's top four officials: the president, the prime minister and the speakers of the two houses of parliament. But the president already enjoys a broad measure of immunity and neither of the two speakers has asked for it, so the law is in effect tailor-made for Berlusconi.

Its approval led to the suspension of charges against the prime minister in two trials: one for allegedly bribing David Mills, the husband of British Cabinet Office minister Tessa Jowell, to give false evidence; the other for alleged tax evasion.

Prosecutors in the second trial appealed to the constitutional court, arguing the immunity act violated principles in the charter of the republic, including citizens' equality before the law.

This is the second time Italy's prime minister has tried to shield himself from the law. A previous immunity act, passed by parliament during his last government, was ruled unconstitutional in 2004.

PM's battles

David Mills case Tessa Jowell's husband was sentenced to 4½ years in February. It is alleged Berlusconi paid a lawyer £387,000 to give false evidence at two trials in the 1990s, during which the Italian prime minister was accused of using offshore companies, set up by Mills, to channel illegal payments. In a letter to his accountant, Mills wrote that he had "kept Mr B out of a great deal of trouble". Mills is appealing against sentence, a case to be heard on Friday.

Tax evasion Mills is also implicated alongside Berlusconi and 12 others in a trial alleging tax fraud. Prosecutors claim Berlusconi used offshore companies to buy American film rights and then sell them on, at an inflated price, to Mediaset, a television company that is part of the Berlusconi group. It is alleged that this shaved €60m off Mediaset's tax bill by reducing profits, and that Berlusconi siphoned off the difference for his children. He is charged with tax evasion, embezzlement and false accounting; David Mills is charged with money laundering.

Fininvest Last week a judge ordered Berlusconi's holding company Fininvest to pay $1bn to a rival for the takeover of the Mondadori publishing house in a case in which three Berlusconi associates were convicted of corrupting a judge to rule in favour of Fininvest. Berlusconi wasn't prosecuted in the case, but the court found he was "co-responsible".

Katy Stoddard