Populist leader Beppe Grillo has said his 5-Star Movement would support individual measures in parliament but would not give a confidence vote to any government led by traditional parties.
Mr Grillo won the most votes in Italy's election this week.
He said on Twitter that 5-Star "will not give any vote of confidence in the [centre left] Democratic Party (PD) or anybody else but will vote in the chamber for laws which reflect its programme".
The centre-left won the most seats in parliament but is well short of a governing majority.
Democratic Party leader Pier Luigi Bersani has made cautious overtures to Mr Grillo's party, which took a huge protest vote in the election.
But the 5-Star leader showed him on his blog as "a dead man talking," suggesting any agreement will be difficult.
Meanwhile, Italian President Giorgio Napolitano has cancelled a dinner with German chancellor candidate Peer Steinbrueck after the centre-left politician called Mr Grillo and former Italian premier Silvio Berlusconi a pair of "clowns".
A spokesman for Mr Steinbrueck said Mr Napolitano, who is on an official visit to Germany, had called off a dinner scheduled for tonight in Berlin.
The spokesman said Mr Steinbrueck understood the Italian president's reasons for cancelling.
Mr Steinbrueck said at a Social Democrat (SPD) rally last night that he was "appalled that two clowns have won" the Italian elections and he made clear he was referring to Mr Berlusconi and Mr Grillo.
Mr Steinbrueck will take on Chancellor Angela Merkel in Germany's next national election in September.