Party officials said a total of 78.5 percent of members who voted in an online poll had backed a proposal for the party's MEPs to seek membership of the Alliance of Liberals and Democrats for Europe (ALDE), most of whose members are ardent enthusiasts for European integration.
ALDE MEPs are to decide on Tuesday evening whether to respond positively to Five Star's overtures and its decision to sever its alliance with the UK Independence Party (UKIP) in the Europe of Freedom and Direct Democracy (EFDD) group.
Party leader and founder Beppe Grillo had recommended the move on Sunday, arguing that Britain's vote to leave the European Union had made UKIP redundant.
Grillo had forged an alliance with UKIP's then leader Nigel Farage after the 2014 elections to the pan-European assembly.
At the time he ruled out joining ALDE, a grouping led by the former Belgian prime minister, Guy Verhofstadt, who is also the EU's chief Brexit negotiator.
The U-turn is likely to be interpreted as a sign of Five Star watering down its own Euroscepticism as Italy heads for elections which must take place in the next 14 months.
Current polls suggest Grillo's party could emerge from the vote with the biggest share of the vote but its reluctance to forge alliances with other parties is an obstacle to it forming a government.
Grillo is a long-standing opponent of Italy's membership of the euro and has called for a referendum on withdrawal from the single currency.
But he does not question Italy's membership of the European Union and his younger lieutenants rarely address the euro issue.