Foreign Minister Gianfranco Fini, leader of the National Alliance (AN), on Tuesday said his party would decide whether to leave the cabinet after Berlusconi addresses parliament on Wednesday, ahead of a confidence vote the next day.
If AN goes through with its threat to become the second party to quit Berlusconi's four-way coalition, it would bring down the government one year before schedule.
Berlusconi has been in crisis since the much smaller Union of Christian Democrats (UDC) quit his government on Friday, demanding he make sweeping changes after the centre-right suffered a bruising defeat in regional elections two weeks ago.
Both the UDC and the AN believed they had secured a promise from Berlusconi to resign temporarily on Monday - a formal move required under Italy's constitution - to form a new cabinet with a fresh policy platform.
But instead of handing his resignation to President Carlo Azeglio Ciampi, Berlusconi told the head of state he would stay and ask for a vote of confidence.
Romano Prodi (L) leads in opinion
polls for the next election
Italian politics now moves into largely uncharted territory, with Berlusconi apparently challenging UDC and AN to either back him or vote him out of office, risking an election as early as June.
Opinion polls indicate that the election would be won by the centre-left led by former European Commission president Romano Prodi.
Both parties say they will support Berlusconi in Thursday's confidence vote, but their future backing looks far more doubtful.
Analysts say Berlusconi would be forced to resign anyway if two of the four government parties left his cabinet.