ROME (Reuters) - The Italian Chamber of Deputies on Thursday gave its final approval to a contested electoral law that is likely to penalize the anti-establishment 5-Star Movement in next year’s election.

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The bill, which is supported by the ruling Democratic Party (PD) and mainstream center-right opposition parties, was approved in a secret ballot by 375 votes to 215, and will now move to the upper house Senate for further debate.

An election is due by May 2018. Analysts say the new system looks highly unlikely to throw up a clear parliamentary majority, with opinion polls showing the center-left, center-right and 5-Star splitting the vote three ways.

The so-called ‘Rosatellum’ law favors parties which group together ahead of the election. The 5-Star refuses to join any alliance and says the reform could cost it at least 50 seats in the next parliament, hobbling its chances of taking power.

The party, which tops many opinion polls, has held two days of street protests against what it sees as an attempt by its mainstream rivals to scupper its chances at the election.

“You are manipulating the foundations of our democratic system and rigging the rules before the game starts,” 5-Star leader Luigi Di Maio told parliament before the vote.

“Every single day of the election campaign we will remind people of what you are doing today.”

Small left-wing and right-wing parties are also against the bill, which is supported by the center-right Forza Italia (Go Italy!) and the right-wing Northern League.

Ettore Rosato, the parliamentary party leader of the PD who has put his name to the reform, said it was the best compromise possible among political adversaries and blamed 5-Star and leftist parties for refusing to join negotiations.

“You always said no to everything that we proposed,” he told the Chamber.

President Sergio Mattarella has called for new voting rules because he says the current system is too different for the upper and lower houses, meaning it could conceivably throw up conflicting majorities.

All previous attempts to harmonize the rules have failed, most recently in June when dissident deputies used a secret vote to up-end part of the proposed legislation.

The 5-Star and other opponents of the new rules had hoped that the same would happen in Thursday’s final vote in the Chamber, which had already approved the bill in three votes of confidence held with open voting.

The package still needs the approval of the upper house Senate, where Gentiloni’s government does not have a stable majority. The upper house is expected to begin discussing and voting on the bill next week.