By Valentina Consiglio and Gavin Jones
ROME (Reuters) - Italy's electoral law is unconstitutional, its top court ruled on Wednesday, piling pressure on political parties to reform a system blamed for creating parliamentary deadlock.
Most politicians agree, at least in public, that the electoral rules which helped produce a hung parliament after February's national vote must change to give Italy a chance of forming a stable government.
But despite repeated exhortations from business leaders, union chiefs and President Giorgio Napolitano, progress on voting reform has long been blocked by parties worried that a new system could damage their electoral chances.
"Now there is no more room for excuses from anyone, we have to move, quickly, to change the law," said Interior Minister Angelino Alfano, whose breakaway group from Silvio Berlusconi's center-right is a key part of the fragile ruling coalition.
The ruling is not retro-active and therefore does not affect the status or validity of the current parliament, a source close to the constitutional court told Reuters.
It may actually reduce the danger of an imminent collapse of Prime Minister's Enrico Letta's coalition because it would be difficult to contemplate dissolving parliament before the voting law has been overhauled.
The constitutional court picked out the "winner's bonus" system where the coalition with the biggest number of votes automatically gets 55 per cent of the seats in the lower house, irrespective of its actual share of the vote.
That can give a political grouping without an overall majority total control of the lower house, but none at all of the upper house, the Senate, which is voted in through a different system.
The resulting wrangling for power can be particularly fierce in a country split into hostile but roughly equal blocs of center-right and center-left with the anti-establishment 5-Star Movement just behind.
The right-left coalition that finally emerged from the February vote is made up of political rivals who remain divided on policy and have struggled to force through reforms that analysts say Italy needs to face up to its financial woes.
The court also ruled against the way voters could only vote for party lists of candidates, robbing them of a chance to select an individual representative in parliament.
FRENCH-STYLE RUNOFF, RETURN TO PREVIOUS LAW?
The practical consequences of the court's decision remain unclear until it publishes its reasoning and the "judicial effects", which it said it would do in the next few weeks.
In the meantime it invited parties to take their own steps.
"Parliament is free to approve new electoral legislation, based on a political choice, as long as it respects the constitution," the court said in a short statement.
A number of possible alternatives have been mooted, including a French-style run-off system with two rounds of voting, or a return to the system in force until 2005.
"I think the most likely thing is that we will go back to the previous law," said Gianfranco Pasquino, political science professor at Bologna University.
That law involved a hybrid system where 75 percent of lower-house seats were allocated by a British-style first-past-the-post system, and the remainder by proportional representation.
However, Pasquino said he believed this would happen by "default", as a result of the court's ruling not due to a political agreement.
"No-one is parliament has the political strength to broker a deal," he said.
The current electoral law was derided even by the conservative minister responsible for its creation and the left have long said it was created to keep them out of power.
(additional reporting by Naomi O'Leary, writing by Gavin Jones, editing by Andrew Heavens)