ROME - Italy's top court ruled on Wednesday that a law granting Prime Minister Silvio Berlusconi immunity from prosecution violates the constitution, in a verdict that could reopen trials against him and undermine his government.

Judicial sources said the Constitutional Court ruled that the law passed last year by the center-right leader, soon after he returned for a third term in power, violates the principle that all citizens are equal before the law.

The immunity law is also invalid because it was passed by parliament as a normal law rather than a constitutional reform, which is harder to approve and can be rejected by referendum, the source said, regarding the Constitutional Court ruling.

It is the second time that the highest court in the land has thrown out Berlusconi's attempts to have immunity from the cases against him, after an earlier version was rejected in 2004.

The immunity also covered the president and two speakers of parliament but it was 73-year-old Berlusconi, who has faced many corruption and fraud accusations linked to his Mediaset business empire, who had most at stake from losing it.

The state attorney acting for Berlusconi had warned that if the law were overturned, the prime minister would become so entangled he would be unable to do his job as premier properly.

His center-right allies had even threatened early elections if what they call concentric attacks on Berlusconi over his private life and business dealings continue, though the premier vowed earlier this week to serve out his full term until 2013.

OPPOSITION CELEBRATES

The opposition celebrated the verdict, with the anti-graft Italy of Values party saying: Berlusconi would be well advised to pack his bags and get a change of air.

But center-right deputy Jole Santelli said the verdict was unfair and poses a problem of political and legal uncertainty in our country, and therefore a problem for our democracy.

The Alfano Law, one of Berlusconi's first acts after winning last year's election, halted all the cases against him, including one where he is accused of bribing British lawyer David Mills to give false testimony to protect his businesses.

Two other cases, one accusing him of tax fraud and false accounting in the purchase of TV rights by his Mediaset group and another alleging he tried to corrupt opposition senators, have also been frozen. Berlusconi denies any wrongdoing.

Prosecutors in those cases appealed to the Constitutional Court.

The ruling comes at a time when the premier's normally high approval ratings have been eroded by a series of sex scandals, including prostitutes attending parties at his home -- one of whom went public with some explicit recordings.

The center-right plans a mass rally in coming weeks to show solidarity with Berlusconi in the face of mounting opposition, which Berlusconi says has been orchestrated by the left-wing press in Italy and by biased Italian magistrates.

(Additional reporting by Paolo Biondi; writing by Stephen Brown)