By Valentina Accardo
PARMA, Italy (Reuters) - The mayor of Parma quit Italy's anti-establishment 5-Star Movement on Monday in the latest sign of turmoil within the party that represents the biggest threat to Prime Minister Matteo Renzi.
Federico Pizzarotti's surprise election in the northern city in May 2012 was the first major breakthrough for 5-Star, which had been founded by comedian Beppe Grillo just three years earlier.
However, Pizzarotti's relations with the movement's national leadership cooled quickly after he rowed back on a campaign promise to close the city's rubbish incinerator.
One of 5-Star's most independent voices, 42-year-old Pizzarotti won plaudits for his overall management of Parma, a city of around 200,000 people, but he became increasingly isolated within the movement.
He was suspended from the party in May after he announced he was under criminal investigation over the way he appointed the head of the city's opera house. Even after he was cleared last month the party did not reinstate him.
"As a free man I have no choice but to leave this movement and what it has become today," Pizzarotti said at a news conference, adding that none of the party's senior figures had even telephoned him since his name was cleared.
He said he would not resign as mayor and was considering running either as an independent or with another party when his mandate expires next year.
5-Star has been racked by internal divisions ever since it won a stunning 25 percent of the vote in its first national election in February 2013, with about a fifth of its parliamentarians defecting within 18 months.
The infighting has never eroded its support for long, however, especially among the millions of young people who see it as the only "clean" alternative to traditional parties that have often been tainted by corruption.
5-Star won another big breakthrough in June, when its young female candidates were elected as mayors of the capital Rome and the northern industrial city of Turin.
While the party appears to be running Turin with little difficulty, it has come badly unstuck in Rome, where the mayor's first three months in office have been blighted by in-fighting and resignations.
Polls show that 5-Star's travails in Rome have modestly dented its support but that it remains the most likely winner of national elections due in 2018 under the two-round voting system introduced by Renzi last year.
(writing by Gavin Jones)