By Crispian Balmer
ROME (Reuters) - Italy's constitutional court on Wednesday threw out aspects of an electoral law approved by former prime minister Matteo Renzi but presented a reworked version that can be used immediately, raising the chance of early elections this year.
Italy's largest parties - Renzi's Democratic Party (PD) and the anti-establishment 5-Star Movement - are both calling for a vote by the summer, about a year ahead of schedule.
The system laid out by the court, which only applies to the lower house Chamber of Deputies, is based on proportional representation and hands a clear parliamentary majority to any
party winning 40 percent of the vote.
Both of these aspects were part of Renzi's law, but the court also said the election should be held in just one round, eliminating the run-off between the two largest parties which Renzi had envisaged if none got 40 percent in the first round.
No opinion polls put any of Italy's plethora of parties anywhere near 40 percent, meaning the new system will probably lead to a coalition government.
That may benefit traditional parties, including the PD, and penalize 5-Star, 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 there are now different voting systems in the lower house and the upper house Senate.
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.
Renzi quit office in December after losing a referendum on a planned overhaul of the constitution, and Wednesday's ruling was another blow to his legacy. He presented his electoral law, approved in 2015, as a landmark reform, but it was never used.
Within minutes of the court's verdict, 5-Star called for an immediate election, saying its objective was to win 40-percent.
"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 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; Editing by Toby Chopra)