The rebel deputies, three of whom have already left Berlusconi's crumbling coalition, wrote to the premier saying Italy needed a "new political phase and a new government.
"Be the backer of a new political phase and a new government," the deputies wrote, according to the letter published in the daily Corriere della Sera. One of the rebels had hitherto been seen as among Berlusconi's closest supporters.
Berlusconi has rejected calls to stand aside and make way for an interim government, saying the only alternative would be to hold early elections next spring, a step he says would be irresponsible while the crisis continues.
A government source told Reuters that Berlusconi had informed his European partners at a G20 summit in Cannes on Thursday that he would call a confidence vote within 15 days on new measures to face the economic crisis.
But he could fall before then if government rebels join the opposition in an important vote Tuesday to sign off on the 2010 budget.
As the number of party rebels grew, another deputy in the ruling PDL, Giuliano Cazzola, gave an interview to the online Affaritaliani daily saying Berlusconi should leave and allow another center-right government to take power.
"The government should resign and the PDL should manage a different solution without clinging to the alternative 'Us or new elections'," he said, suggesting that Berlusconi's chief of staff Gianni Letta could lead a new administration.
President Giorgio Napolitano said Tuesday he was sounding out support for reform from political forces outside the ruling center right, suggesting he was contemplating the possibility of a broad-based national unity government.
But in a statement Thursday he said the ruling coalition had insisted Berlusconi could continue, there was no alternative to him, and he could carry through on his commitment to economic reform. On the other hand, opposition leaders wanted a unity government, Napolitano said.
BOSSI OPPOSES TECHNOCRAT GOVERNMENT
Umberto Bossi, leader of the devolutionist Northern League and Berlusconi's key ally, confirmed his opposition to a technocrat government after talking to Napolitano and said the party would prefer early elections, a year ahead of schedule.
The head of state does not have the power to dismiss a government with a parliamentary majority but as growing numbers of PDL deputies desert Berlusconi, the opposition believe they could have the numbers to topple him as early as next week.
With doubts over Greece's future in the euro zone already causing havoc in the markets, the renewed political uncertainty in Rome racked up pressure on Italian government bonds.
Yields on 10-year BTP bonds hit more than 6.3 percent, creeping closer to the level of 7 percent which many analysts believe could lead to a so-called "buyers' strike" where investors take fright and refuse to buy the paper.
The risk premium over benchmark German Bunds rose at one point as high as 462 points, the widest spread since 1995, reflecting the growing worries about Italy, the euro zone's third biggest economy.
With Greece teetering on the brink of possibly leaving the euro, the future of Europe's single currency could now depend on preventing a meltdown in Italy, which would overwhelm the bloc's current defense mechanisms.
Berlusconi, struggling to contain divisions in his center-right coalition, failed to win support at a cabinet meeting late Wednesday for the comprehensive reforms to stimulate growth and cut Italy's massive debt that he wanted to take to the G20 meeting.
His supporters accused Economy Minister Giulio Tremonti, a constant thorn in his side, of blocking a deal.
Instead of a decree that could have been put into action immediately, the meeting merely agreed on a so-called maxi amendment, containing a number of measures to add to a budget bill currently before the Senate. The confidence vote will be on this bill.
A government statement said the amendment was in line with what had been agreed with EU partners at a summit last week but contained no details.
An official said the package included tax breaks for infrastructure investment, simplifying bureaucracy and helping youth employment though apprenticeships.
"Italy can absolutely manage this situation and save itself if it does its work properly," said Corrado Passera, head of Intesa San Paolo, Italy's biggest retail bank, adding that it required a government "which acts differently from this one."
Rome's borrowing costs have been capped since August by the European Central Bank's bond-buying program. But as the crisis has spread and concerns about Italy's towering public debt have grown, ECB intervention has become less and less effective.
Market concern about Italy was underlined by French bank BNP Paribas, which reported Thursday that it had slashed its sovereign exposure to Italy by 8.3 billion euros, or 40 percent.
Berlusconi has rejected a growing chorus of calls to step aside, from groups ranging from the center-left opposition to business and banking associations, unions, the Catholic church and now rebels in his own party.