* 5-Star Movement consults its members on coalition deal
* Online vote is risky due to deep-rooted antagonism to PD
* Web-based direct democracy is founding principle of party
* Critics say votes are opaque, undemocratic
By Gavin Jones
ROME, Sept 3 (Reuters) - Efforts to form a new Italian government could be dashed later on Tuesday, depending on the outcome of an online vote.
Earlier in the day, the anti-establishment 5-Star Movement and centre-left Democratic Party (PD) unveiled a shared policy agenda to serve as the basis of a new ruling coalition, setting aside years of political hostility.
However, the deal faces a final hurdle: the 5-Star party is submitting the accord to a web-based ballot of its members, respecting the party's credo of direct democracy. Voting has already opened and will conclude at 6 p.m.(1600 GMT).
WILL 5-STAR MEMBERS BACK THE DEAL?
The outcome is far from certain. Many 5-Star members view the PD as the very symbol of a corrupt Italian establishment that 5-Star was set up to fight a decade ago.
The question is quite neutral: "Do you agree that the 5-Star Movement should form a government together with the Democratic Party, led by (Prime Minister) Giuseppe Conte?"
The online platform received a record high 30,000 votes in the first two hours of balloting, the group said on Facebook. In all, 115,372 registered supporters are eligible to take part.
An opinion poll by the Winpoll agency on Sunday showed 43% of 5-Star voters backed an alliance with the PD. While less than a majority, that was still up steeply from previous polls and twice as much as for the other options on the table.
Some 22% wanted a snap election, 16% favoured resurrecting 5-Star's previous coalition with the hard-right League party, and 19% were undecided.
Online votes have normally followed the will of 5-Star's top brass, but not always. In 2014, members voted against scrapping a law making irregular immigration a criminal offence, though the party's founder had wanted to revoke the legislation.
5-Star's chiefs are divided over the proposed coalition with the PD and some analysts say the online vote gives the party a last-minute way out if it feels it is being outmanoeuvred.
HOW DOES THE VOTE WORK?
The vote will be held on 5-Star's internet platform, dubbed Rousseau after the 18th century Swiss-born philosopher.
Web-based direct democracy has been one of 5-Star's core principles since it was founded in 2009 by comedian Beppe Grillo and Gianroberto Casaleggio, an internet visionary who died three years ago. In its early years the movement used its website and Grillo's blog to debate and hold votes before Rousseau, a purpose-built platform, was developed and wheeled out in 2016.
IS THE VOTING PLATFORM SECURE?
The system has been plagued by hacking attacks during key votes, and in April this year Italy's data protection authority fined the company that runs the platform 50,000 euros ($55,720.00) for failing to protect users' personal details.
"I don't feel comfortable because there is no clearly identified third party that controls the votes and vouches for their authenticity," says David Puente, a computer expert and web developer who used to work for Casaleggio Associati, the company that ran 5-Star's original website.
Critics also say that deciding strategy by online votes involving just tens of thousands of activists runs counter to parliamentary representative democracy. 5-Star says some 100,000 members are enrolled on Rousseau, but fewer than half usually take part in its votes.
In a statement Casaleggio Associati said the voting platform had been updated five months ago with the latest security technology.
"The criticism dates back to the first version of the Rousseau platform and, like all innovations of this type, it can be improved. The system can manage in absolute security a very high level of requests. Since nine o'clock this morning more than 40,000 members have voted," the statement said.
($1 = 0.8973 euros)
(Reporting By Gavin Jones, Angelo Amante and Crispian Balmer Editing by Mark Bendeich and Jon Boyle)