Italian premier Silvio Berlusconi's immunity from prosecution in two unfinished court cases could be overturned by a judicial hearing that opened today.

His lawyers told the hearing that the law shielding him complies with the constitution.

Mr Berlusconi is a defendant in two trials in Milan on corruption and tax fraud charges. Both proceedings have been suspended thanks to the legislation, but will resume if the law is overturned by the Constitutional Court.

Mr Berlusconi has always denied wrongdoing.

The law has drawn accusations it was tailor-made to protect Mr Berlusconi from his trials.

The hearing before the court, held inside an 18th-century palazzo in the heart of Rome, lasted about an hour. The 15-judge court will begin deliberations on Thursday morning, according to president Ugo De Siervo, and a decision is expected the same day.

The ruling was sought by the Milan tribunals that are trying Mr Berlusconi.

The law in question marked the third attempt at shielding Mr Berlusconi during his various stints as head of government. In both previous cases, the court struck down the immunity legislation, in 2004 and 2009.

Mr Berlusconi stands accused of bribing British lawyer David Mills in order for him to lie in court in the 1990s to protect Mr Berlusconi's interests.

Mills was convicted in 2009 of having taken a 600,000 dollar bribe, but the guilty verdict was overturned last year when Italy's highest criminal court ruled the statute of limitations had expired.

In the other trial, Mr Berlusconi is accused of tax fraud in the purchase of TV rights by his Mediaset broadcasting empire.

Amid protests from the opposition, the legislation was passed in March by Mr Berlusconi's conservative forces, who control parliament, and went into effect the following month.

The legislation suspends court proceedings for up to 18 months if the defendant has a "legitimate impediment" stemming from being premier or a member of government. It was designed as a stopgap measure to buy the conservatives time while they prepare a more thorough constitutional amendment granting full immunity to top officials.

Backers insist the measure is needed because Italians voted for Mr Berlusconi to govern, not be distracted by court cases.

The Constitutional Court needs to decide if the measure complies with the principle that all are equal under the law and that it meets the necessary formal requirements.

Mr Berlusconi's lawyers told the court that the measure was necessary to protect the right to a fair defence.

lawyer Nicolo Ghedini said that Italian law allows for other ways to postpone trials, such as grave illness, and also argued that the law does not infringe on the right to a speedy trial, noting that postponements of several months are typical in Italian courts.

"Nobody can state that the principle of speedy trial is respected in our country," he said of Italy's notoriously slow justice system.

Mr Berlusconi's lawyers also insist the legislation only provides a temporary shield because of the 18-month limit and falls short of granting a full-fledged immunity. The Milan judges say the law creates a de facto immunity.

The court could reject the law outright, or back it, which would be a victory for Mr Berlusconi because his trials would remain suspended.

Other possible scenarios envisage compromises: One would have the Constitutional Court assign individual judges to decide whether Mr Berlusconi has a legitimate impediment for each trial.

"A declaration of unconstitutionality of the law would be a very hard (blow) for the government, not only for the fact that the trials will go on automatically, but also for the symbolic meaning of this," said Chiara Magrini, a legal expert and professor at John Cabot University.

Mr Berlusconi has a history of legal problems and makes no secret of his contempt for what he says are left-leaning prosecutors and judges bent on driving him from office.

He has suffered a turbulent period. Weakened by sex scandals, he barely survived two confidence votes last month to cling to power, but his majority in parliament remains shaky.