By Crispian Balmer
ROME (Reuters) - Italy's constitutional court on Wednesday approved a new voting system based on proportional representation that raises the chance of an early election this summer, as demanded by the two biggest parliamentary parties.
The Constitutional Court ruled that any party winning 40 percent of the vote would be awarded a clear parliamentary majority and that the election should be held in just one round.
No opinion polls put any of Italy's plethora of parties anywhere near 40 percent, which means the new system, which only applies to the lower house, will probably lead to a coalition government.
That may benefit traditional parties, including the ruling Democratic Party (PD), and penalize the main opposition 5-Star Movement, which has always refused to form alliances.
The court said the amended law could be used immediately if elections were called, even though following its ruling the two houses of parliament now have two different voting systems.
However, President Sergio Mattarella is the only one with the power to dissolve parliament, and he has said a vote should not be held until the systems are harmonized, in order to try to ensure political stability.
A vote is not scheduled until 2018 but former Prime Minister Matteo Renzi, who heads the PD, has said he wants one in June.
Renzi quit office in December after losing a referendum on a planned overhaul of the constitution. Wednesday's ruling was another blow to his legacy, with the court scrubbing sections of the electoral law that he had presented as a landmark reform.
Within minutes of the court's verdict being released, 5-Star also called for an immediate election, saying its objective would be to win 40-percent backing nationally.
"We will present ourselves at the next election without making any alliances," founder Beppe Grillo wrote in his blog.
He said parliament could swiftly revise the Senate law in line with the court ruling, rather than keeping a system geared towards parties forming coalitions ahead of elections.
The lower house leader of Renzi's PD told Reuters it was willing to strike a deal with other parties to make the two voting systems compatible, but said that if no consensus was found, then elections should still be held without delay.
Former prime minister Silvio Berlusconi's Forza Italia (Go Italy!) party, which is trailing in the polls, struck a more cautious note, stressing parliament needed to work out a deal.
"Since they are totally divergent (voting systems), there needs to be a great deal of work in parliament to harmonize them and guarantee governability," parliamentary party leader Renato Brunetta said.
Prime Minister Paolo Gentiloni, who is close to Renzi, said at the weekend he was confident a new election law could be in place in time for a vote in the summer or autumn.
(Additional reporting by Gavin Jones, Steve Scherer and Massimiliano di Giorgio)