Italian Prime Minister Romano Prodi has held crisis talks with members of his cabinet after the departure of an ally threatened to topple his coalition.
Mr Prodi (L) had the support of Mr Mastella (R) until last week
After the talks, Mr Prodi asked to address the lower house of parliament, the Chamber of Deputies on Tuesday.
There was speculation Mr Prodi would resign after his former justice minister called for early elections.
Clemente Mastella resigned last week after his name - and that of his wife - were mentioned in a corruption probe.
His centrist Udeur party has three seats in the upper house and its withdrawal has cost Mr Prodi his Senate majority of one.
On Monday, Mr Mastella told reporters Mr Prodi's coalition was "finished".
He said he would oppose the government in any vote of confidence.
The BBC's Christian Fraser reports from Rome that is still possible for Mr Prodi to govern with the support of several unelected life senators but the question is - for how long?
Mr Mastella had promised last week to continue supporting Mr Prodi on a case-by-case basis.
We are for elections and if there is a confidence vote in the future we will vote against the government
Leader of Udeur party
But the ex-minister, who at times has been accused of holding the government to ransom, said the experiment with the centre-left was now over, our correspondent says.
"We are for elections and if there is a confidence vote in the future we will vote against the government," Mr Mastella said.
On Wednesday, Environment Minister Alfonso Pecoraro Scanio faces a vote of no confidence in the Senate for his handling of the rubbish crisis in Naples.
Defeat for him would undermine Mr Prodi's position despite the majority his coalition enjoys in the Chamber of Deputies.
Antonello Soro, the parliamentary chief of Mr Prodi's Democratic Party, has said the coalition is unravelling at the seams.
"I cannot exclude the possibility that at the end of it all we'll be obliged to go to the polls," he said as the coalition met.
Italy's elected Senate is a more powerful body than upper houses in some other countries.
Under the constitution, any government must "obtain the confidence" of both it and the Chamber of Deputies.