Italy's constitutional court ruled on Thursday that a law which has protected Prime Minister Silvio Berlusconi from prosecution for corruption and tax fraud was invalid in key points and judges could order him to stand trial.

The court said in a brief statement that elements of the law, which allowed cabinet ministers including Berlusconi to claim exemption from trial because their official duties prevented attendance, violated the constitution.

The ruling by the 15-member court will further weaken the 74-year-old premier, who is battling to shore up his fragile centre-right government after scraping through a no-confidence vote last month.

The judges struck down the blanket exemption offered by the so-called legitimate impediment law and ruled that it would be up to individual trial judges to decide on a case-by-case basis whether ministers would have to appear.

Basically the court has completely rejected the argument of the government and the premier's advisers, Felice Casson, a former judge who is now a senator for the opposition Democratic Party said in a statement.

The ruling had been sought by judges in Milan, where Berlusconi faces three trials over alleged offences connected with his Mediaset broadcasting empire.

He is accused of bribing his lawyer David Mills to give false testimony and also faces charges of embezzlement and tax fraud in connection with the sale of television rights.

Whether the premier, who rejects the accusations, comes before the judges is uncertain, however, given the possibility that proceedings may be delayed until the statute of limitations means the cases are shelved because the charges go back too far.

With his government clinging on to power after a split with former ally Gianfranco Fini last year, some commentators had speculated that an unfavourable decision could prompt a fresh crisis and bring early elections.

NO DANGER TO GOVERNMENT STABILITY

Berlusconi, the billionaire owner of Italy's biggest private broadcaster, has been in conflict with judges since he launched his political career in 1994 and he says he has faced more than 100 different court cases.

Before the decision, he rejected the charges against him and brushed off any suggestion that the ruling could threaten his coalition administration.

There is no danger to the stability of the government, whatever the outcome of the constitutional court's decision, he told reporters on Wednesday.

His lawyers issued a statement on Thursday saying the court appeared to have misunderstood a measure intended to allow the government to conduct business without the distraction of legal battles but said the decision should be respected.

Ministers said Berlusconi was the victim of a justice system out of control.

The real anomaly in Italy is certainly not Berlusconi, it's those who try to overturn the choice of Italians through actions which have nothing to do with politics, Education Minister Mariastella Gelmini said in a statement.

Whether the ruling triggers a crisis will depend in large part on the attitude of his coalition partners in the Northern League whose support is essential to the government's survival.

The party, based in the prosperous north of Italy, wants a complicated reform of tax laws to give more autonomy to the regions but after that, there has been widespread speculation that it may wish to press for elections.

Looking at the dates facing the government and the majority in the immediate future, we have to take one step at a time, said Giacomo Stucchi, a senior Northern League official.