Romano Prodi, who briefly resigned as premier last week, will face a do-or-die vote in the Senate on Wednesday evening, Senate officials said Monday.
Senate whips said the vote would be preceded by a speech from the centre-left premier, scheduled for Tuesday afternoon.
A House confidence vote will take place on Friday at midday.
Prodi quit last week after two 'rebels' helped defeat a Senate motion on the nine-month-old government's foreign policy.
President Giorgio Napolitano rejected the resignation on Saturday after two days of consultations with political leaders, instead asking Prodi to test his majority via confidence votes in the House and Senate.
The House vote will present no problems for Prodi since he enjoys a solid majority there but the Senate ballot is likely to be another knife-edge affair.
Prodi holds only one more Senate seat than the Silvio Berlusconi-led opposition.
The nine parties in his Communists-to-Catholics alliance have renewed their pledge of loyalty, accepting a new 12-point "non-negotiable" programme drawn up by Prodi, but there are still fears that individual senators could go their own way.
The premier can, however, count on the extra support of a majority of the chamber's seven life senators.
Dubbed the "magnificent seven", the life senators hold the balance of power and have come to the government's rescue several times in the past.
A top opposition moderate, Senator Marco Follini, has also said he will support the government.
DISSIDENT SENATORS STILL A PROBLEM FOR PRODI.
Political analysts expect Prodi to win the confidence vote but say he could still remain at the mercy of his own senators in the long run, particularly over foreign policy.
Prodi, whose first, 1996-98 government was brought down by a Communist ally, has angered hard-leftists and pacifists by refusing to withdraw Italian peacekeeping troops from Afghanistan and approving the expansion of a US military base in the northern city of Vicenza.
The two leftists who helped topple Prodi last week, Franco Turigliatto and Fernando Rossi, have already said they cannot guarantee their support in a crucial ballot next month on refinancing the Afghan mission.
Turigliatto said on Monday that "I feel responsible towards the coalition I was elected with... but on issues such as Afghanistan, I am not ready to compromise".
The senator even backpedalled on his former promise to vote for Prodi in the upcoming confidence vote, saying "I'm still thinking about it and will decide tomorrow. I have doubts about lots of points in Prodi's new programme".
The first article in the programme commits coalition parties to accepting the government's foreign policy, including the continuation of the Afghan mission.
Another key clause is that the premier has the final word in the event of a row.
But at least three other senators remain unconvinced over Afghanistan, including Franca Rame, the wife of Nobel literature prize winner Dario Fo.
The government has been trying to broaden its Senate majority by luring over opposition centrists such as Follini, who was a deputy premier in Berlusconi's administration.
With Follini's vote, Prodi can in theory count on 157 seats in the Senate, two more than the opposition.
Four of the seven elderly life senators are also sympathetic to the centre left: former presidents Carlo Azeglio Ciampi and Oscar Luigi Scalfaro, ex-Christian Democrat premier Emilio Colombo and Nobel-winning scientist Rita Levi Montalcini.
A fifth life senator, seven-time former premier Giulio Andreotti, also said he would vote for Prodi on Wednesday.
Andreotti and two other life senators - former president Francesco Cossiga and auto-styling legend Sergio Pininfarina - contributed to last week's crisis by not supporting the government in the Senate vote.
Andreotti told the press on Monday that his action was in protest at the government's plan to give certain rights to cohabiting heterosexual and same-sex couples.
The veteran Christian Democrat statesman told the La Repubblica daily he was pleased that this bill, which is fiercely contested by the Catholic Church, had been dropped from the new programme.
Meanwhile, Scalfaro warned that a "political problem" would be created if the government survived Wednesday's vote thanks to the life senators.
"The majority has to be self-sufficient, aside from the life senators," the former head of state said.
He also rapped the governing coalition for its constant squabbling and lack of communication skills, saying that "up until now, there has been a serious lack of composure".
Prodi is hoping to remedy this problem with the appointment of a government spokesman, Silvio Sircana.
Sircana was confident on Monday that Prodi would pass the Senate test.
"It (the crisis) is just a pit-stop... The government has cleaned its engines and is ready to go again," he said.
But Berlusconi's Forza Italia party said that "the Prodi government is already dead, divided on everything from foreign policy to gay rights... For Italy's sake, Prodi should go right now and not wait for them to stab him in the back another time".
