Beppe Grillo proposed, in a blog posting, that his anti-euro party instead align itself with a pro-EU group, drawing shock from supporters who have until Monday to vote on the question.
“The recent European developments, like Brexit, have led us to rethink the nature of the EFDD (Europe of Freedom and Direct Democracy) group,” Grillo wrote, referring to its parliamentary grouping led by ex-UKIP head Nigel Farage.
“To remain a member of the EFDD is to face the next two-and-a-half years without a common political objective,” Grillo added.
UKIP is the single largest source of the grouping's members, raising the possibility it could fall apart once British MEPs leave the legislature at the end of their terms in 2019.
There can be no British members of the European parliament once Britain completes its planned exit from the bloc.
The political groupings are important because not only do they provide more political visibility and the possibility of chairing various committees, but also up to 30 million euros ($32 million) in funding during the parliament's five-year term.
Grillo has raised the idea of joining up with the Alliance of Liberals and Democrats for Europe (ALDE), which supports the common currency and European integration.
If M5S sign up with the ALDE grouping, it would become the third largest political force in the European Parliament, behind the centre-right European People's Party (EPP) and the Socialists & Democrats (S&D), Grillo wrote.
Many Five Star supporters reacted with outrage to the proposal, with one writing on Grillo's blog, “ALDE? Are we ultra-liberals or pro-Made in Italy?”
“If the base chooses to join ALDE there will be an endless haemorrhage of votes,” wrote another backer, Alessandro Gasparri.
Matteo Salvini, who leads Italy's anti-immigrant, anti-EU Northern League party, called the proposal an “incredible Europeanist about face by Grillo!”
For a little more power in parliament “the five stars are abandoning a eurosceptic group to join ALDE, the group most in favour of a Europe belonging to the euro, banks, lobbies and immigration,” he added.
M5S members can cast online ballots on Grillo's proposal until midday (1100 GMT) on Monday.