ROME (Reuters) - A shock vote by members of Italian Prime Minister Paolo Gentiloni’s coalition is threatening the stability of his four-month-old government and to stymie negotiations for a new electoral law.

FILE PHOTO: Italian Prime Minister Paolo Gentiloni looks on as he waits for the arrival of Britain's Prince Charles at Chigi Palace in Rome, Italy, April 5, 2017. REUTERS/Remo Casilli

Members of the coalition broke ranks in a secret ballot to elect a member of another party to a key parliamentary committee on Wednesday. It was Gentiloni’s worst setback since he took office in December and bode poorly for the future, political commentators said.

La Stampa daily spoke of a “wind of crisis” buffeting the government that could affect its staying power until the next scheduled national elections next year. La Repubblica said the result had left the government “trembling” and “on the edge of a crisis”.

Gentiloni has been struggling to keep his faction-ridden Democratic Party (PD) together since he took office, and the setback could affect the timing of agreement on a new electoral law ahead of the next national polls.

“Full-blown government crisis?,” asked Wolfgang Munchau, an analyst writing in the UK-based Eurointelligence Professional Briefing. “The vote threatens the end of the coalition ... and could lead to further political fragmentation.”

Ruling coalition members had agreed ahead of time to vote for Giorgio Pagliari as head of the Chamber of Deputies constitutional affairs committee but he lost by a large margin in the secret ballot to Salvatore Torrisi, of the Popular Alliance, a party headed by Foreign Minister Angelino Alfano.

The tally, made public just after it was taken, meant as many as five members of Gentiloni’s coalition, or about one-fifth of the members, secretly voted with the opposition 5-star movement and former Prime Minister Silvio Berlusconi’s Forza Italia party.

The post of head of the constitutional affairs committee is important because it would play a key role in changing the country’s electoral law before the natural end of the legislature next year.

“It appears that a coalition of small parties and PD dissidents are trying to block an electoral reform that could see their influence greatly reduced,” Munchau said.

Former prime minister Matteo Renzi quit in December after suffering a heavy defeat in a referendum on constitutional reform.

He has called for a leadership contest in the PD and supports early elections, arguing the debt-laden country needs a new political impetus. The PD is due to hold primaries for a new leader on April 30 and a congress in May.

The anti-establishment 5-Star Movement, which among other things wants a referendum on membership of the euro currency, is also eager for early elections, with opinion polls showing it neck-and-neck with the center-left PD, well ahead of other opposition parties.