A senior parliamentary figure in Italian Prime Minister Matteo Renzi’s ruling centre-left Democratic Party (PD) resigned early yesterday in protest over his proposed new electoral law, underlining unrest among government backbenchers.

Roberto Speranza, the PD’s lower house parliamentary floor leader, told a meeting of the party late on Wednesday that he was stepping down because he disagreed with government policy.

“I will be loyal to my group and to my party but I want to be just as loyal to my deep convictions,” he was quoted as saying by Italian media.

Speranza is one of a group of around 100 centre-left deputies who have resisted plans to overhaul the electoral system, several of whom also walked out of the meeting in Rome.

Speranza’s resignation as floor leader will not in itself affect Renzi’s ability to govern and it is unclear whether party rebels will vote against the government in Parliament. But the resignation underlines the deep resistance the Prime Minister faces from many elements of the PD.

I will be loyal to my party but I want to be just as loyal to my deep convictions

Although the European Central Bank’s bond buying programme has calmed financial markets in recent weeks, a political crisis in Italy would rattle a eurozone already on edge over the standoff over Greece’s debt negotiations.

The left wing of the PD has clashed repeatedly with Renzi since he came to office just over a year ago, pledging sweeping reforms of the economy and the political system and vowing to take on vested interests in his own party.

Renzi has been pushing to change voting rules to ensure a clear winner in elections since he took over leadership of the PD in 2013, following a deadlocked election which forced the rival centre-left and centre-right blocs into an unwieldy and short-lived coalition.

The proposed electoral law, dubbed the “Italicum”, would introduce a two-round voting system and theoretically avoid the sort of deadlock seen after the 2013 election.

However PD rebels are unhappy about provisions that would let party bosses handpick the lead candidates, worried they would concentrate too much power with the leadership.

The electoral law reform, which successive governments have promised but failed to implement, has been one of the thorniest issues facing Renzi, who came to power following a party coup which deposed his predecessor Enrico Letta.

He has vowed not to back down, saying the fate of his government was tied to the reform and threatening early elections if he cannot get it through Parliament.

Opinion polls show that, despite the dissent in the PD, Renzi has the strongest support of any Italian politician, helped by the even more bitter divisions that have weakened the centre-right Opposition led by former Prime Minister Silvio Berlusconi.

Meanwhile Berlusconi is now free to travel abroad after a court in Milan declared on Tuesday that he had fully served his sentence for tax fraud, although he will still be prevented from running for election.

A judicial source said the court had filed the declaration that Berlusconi’s sentence had been completed, meaning his passport will be returned to him.

However, he remains ineligible for public office under an anti-corruption law passed by the last government.

Berlusconi was given a four-year jail sentence for his part in a tax fraud scheme at his Mediaset broadcasting empire but he was only ordered to perform a year of community service at an old people's home, a period he completed last month.

With regional elections due next month, the end of his sentence should give him more freedom to campaign with his Forza Italia party, which has been split by infighting in recent months as potential successors for leadership of the centre-right jockey for position.

The tax fraud sentence hastened Berlusconi’s political decline by forcing him to quit his seat in the Senate and adopt a much lower public profile than he previously enjoyed as one of Europe’s best-known leaders.