Italy has lurched closer to a crisis since Berlusconi, a partner in Prime Minister Enrico Letta's coalition government, was sentenced last month to four years in prison, commuted to a year under house arrest or in community service, for tax fraud.
Berlusconi's allies made their latest threat to bring down the government, saying they would resign if a special Senate committee meeting next week voted to strip the 76-year-old media tycoon of his seat in the upper house.
Mr Letta has said Italy needs political stability while it struggles to emerge from more than two years of recession.
Mr Napolitano, who had been due to attend a conference on post-war premier Alcide De Gaspari, sent a message to organisers excusing himself and saying "a sudden political development that is institutionally disturbing" required his attention.
How serious a threat the latest move presents is difficult to assess given a series of contradictory signals from Berlusconi's allies in parliament, who are divided between a faction of hardliners and more conciliatory doves.
But Mr Letta's centre-left Democratic Party said the threats could undermine the government.