Mr Berlusconi's opinion poll ratings have dropped recently

Parliament's lower house approved a return to a form of proportional representation last used 12 years ago.

Many opposition deputies boycotted the vote, saying Mr Berlusconi was changing the rules to boost his poll prospects.

The electoral reform bill will next go to the upper house, which is likely to pass it into law, correspondents say.

Mr Berlusconi enjoys a solid majority in both houses of parliament.

Instability fears

According to the BBC's David Willey in Rome, the passing of the bill was preceded by three days of bitter debate in the lower house.

Left-wing lawmakers have accused Mr Berlusconi of altering the law to suit his re-election prospects, at a time when his opinion poll ratings have plummeted.

They also say the law will condemn Italian politics to a future of instability by reviving a flawed system abandoned more than a decade ago.

For much of the last 50 years, Italy had a series of revolving- door governments that were the result of continual squabbling between too many small parties and the coalition leaders.

Prime Minister Berlusconi has argued that the new system is more democratic and offers a more faithful representation in parliament of the power of various parties.