* Anti-establishment 5-Star is main opposition party
* Support dips after surging into parliament in Feb.
* Defectors criticise Grillo; don't mention expenses
By Paolo Biondi and Steve Scherer
ROME, June 7 (Reuters) - Two Italian lawmakers have quit the anti-establishment 5-Star Movement, a sign of trouble within the group that swept into parliament for the first time in February with a quarter of the national vote.
The defections come ahead of this weekend's municipal elections in Sicily, where a regional vote last year catapulted the 5-Star Movement into the national political arena after it captured more than 18 percent of the ballots.
The movement, established by comedian and blogger Beppe Grillo, looked like a potential kingmaker in February's inconclusive general election, but it stuck to a pledge not to form a government with existing parties which it says are all corrupt.
Five-Star flopped in local elections last week, failing to win in any of 564 towns and cities that voted, and support has fallen below 20 percent nationally, an SWG poll published on Friday showed.
Lower house deputies Vincenza Labriola and Alessandro Funari, the first parliamentarians to abandon the movement, criticised Grillo in a statement explaining their defection.
"Decisions have been forced on us from above in the past few months and this broke the trust that had tied us all together in the same dream," said the statement, posted on Labriola's Facebook page.
While 5-Star's philosophy is based on direct democracy through online voting, most big decisions have been made by Grillo, who is not in parliament, and announced on his blog without consulting his deputies.
In a blog entry on Friday, Grillo said parliament was useless because Prime Minister Enrico Letta's right-left coalition government was making policy without consulting lawmakers. Five-Star is the main opposition force.
"Parliament could close tomorrow and no one would notice," Grillo wrote. "It's an empty can of tuna."
There have been repeated media rumours of impending 5-Star defections since a group that included Funari and Labriola rebelled against Grillo's orders to hand back a portion of their generous parliamentary expenses.
"They were among those who stated clearly that they did not want to hand back any part of the allowance," Riccardo Nuti, the 5-Star lower house speaker, said in a radio interview.
The allowance was not mentioned by either Funari or Labriola in their statement. The two will remain in parliament.
The two defections and an expulsion earlier this year of a lawmaker who defied Grillo's veto to appear on TV talk shows leaves the movement with 107 lower-house deputies in the 630-seat chamber and 53 members of the upper house out of a total of 315 senators. (Editing by Robin Pomeroy)
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