The leader of Italy's anti-establishment 5-Star Movement ruled out joining a coalition with Silvio Berlusconi's Forza Italia party on Tuesday, a day before formal government consultations begin.
Until now, 5-Star had said it was ready to talk to all parties after the March 4 national election ended in a hung parliament.
In an interview recorded by La7 TV channel and not yet broadcast, Luigi Di Maio said 5-Star was open to talks with the center-left Democratic Party (PD), though not to its former secretary Matteo Renzi, and the far-right League, but not with Forza Italia, two 5-Star sources said.
"We will propose a government contract like the Germans do and what is written is done, and what is not written does not get done," Di Maio told 5-Star lawmakers on Tuesday according to a statement.
The move will heighten tensions between the League and Forza Italia, which are partners in a center-right coalition that won the most seats in parliament. Forza Italia had no immediate comment.
"The coalition that won the most votes is the center-right and this is our starting point," League leader Matteo Salvini said on Facebook. "We will dialogue with 5-Star, but reject vetoes or commands."
In the election, the 5-Star followed the center right but was the single largest party, and the PD was third, but no one has enough seats to govern alone.
The 5-Star position will also put pressure on the PD to consider joining a coalition with 5-Star. So far the PD has toed the hard line - set by Renzi before he resigned as party leader in the wake of the vote - to not consider a deal with 5-Star.
"The 5-Star leader's proposal is unacceptable," Andrea Marcucci, the PD's Senate leader, said on Twitter.
Though the sources did not say what reason Di Maio gave for rejecting Forza Italia, the 5-Star has always been hostile to Berlusconi because the party has vowed to clean up Italian politics. Berlusconi has a conviction for tax fraud and is on trial in another case for allegedly bribing witnesses.
President Sergio Mattarella on Wednesday begins formal consultations in which he will try to get rivals to seek common ground and form a government together. He will meet the main parties, including 5-Star, on Thursday.
But more consultations could be held later and the process could take weeks. If no deal can be found, Mattarella may be forced to call a new election.