ROME (Reuters) - Fresh from its successes in last month's local elections, the anti-establishment 5-Star Movement (M5S) is now Italy's most popular party and would easily win power if a national election were held now, opinion polls show.
Three polls this week said M5S had overtaken Prime Minister Matteo Renzi's Democratic Party in voter preferences, reaching around 30 percent of the vote and continuing a long trend of rising support while Renzi's popularity ebbs.
Moreover, under the two-round electoral law pushed through by Renzi last year, 5-Star's advantage increases dramatically to as much as 13 points, the polls show.
One by the Ipsos agency published in daily Corriere della Sera on Wednesday gave M5S 30.6 percent of the vote, up from 28.9 percent in April, while the PD fell to 29.8 percent from 31.1 percent.
A Euromedia survey released on Tuesday and a poll by EMG on Monday gave M5S leads of around 0.5 points over the PD, while a survey by Demos published on Friday put M5S two points ahead on 32.3 percent.
The main center-right parties, the anti-immigrant Northern League and former Prime Minister Silvio Berlusconi's Forza Italia (Go Italy!) trail with around 12-13 percent each.
Italy's next national election is scheduled for 2018.
But all recent polls show that under the new electoral system M5S would easily win in the second round ballot if the election were held now.
The strength of M5S in second-round ballots was reflected in mayoral elections last month, when it won in 19 of the 20 run-offs it contested, including the capital Rome.
This is because M5S has no clear ideological identity, so in the second round left-wing voters support an M5S candidate rather than a right-wing one, and right-wing voters back an M5S candidate over a left-wing one.
There has been wide speculation that Renzi will try to change the electoral law to block M5S, but he has denied this.
The 5-Star Movement, founded by comedian Beppe Grillo in 2009, bases its appeal on the fight against Italy's rampant corruption and pledges to break down the privileges of its political and business elite.
Grillo has taken a back seat over the last year in favor of a new group of young leaders headed by 30-year-old Luigi Di Maio, who is widely expected to be the party's candidate for prime minister at the next election.
Di Maio is now Italy's most popular political leader with an approval rating of 30.6 percent, compared with 27.2 for Renzi, according to the Euromedia survey.
(Reporting by Gavin Jones; Editing by Richard Balmforth)