By Silvia Aloisi

ROME (Reuters) - Italy's government looked on the verge of collapse on Tuesday after an ally of Prime Minister Romano Prodi deserted the centre-left coalition, depriving him of a parliamentary majority.

Prodi was to address parliament at 10 a.m. (0900 GMT) in a bid to hold the government together, but his chances of survival appeared slim as he was likely to lose any confidence vote in the upper house where he now has fewer seats than the opposition.

"We'll need to see if Prodi will throw in the towel -- something that's anything but certain," said leading daily Corriere della Sera. "(But) the hypothesis that Prodi can emerge intact from such a violent blow seems unbelievable."

Prodi's government has wobbled many times since he narrowly beat his centre-right rival, the media tycoon Silvio Berlusconi, at elections in April 2006.

The latest, possibly fatal, crisis followed the resignation last week of justice minister Clemente Mastella and his decision on Monday to withdraw the support of his small Catholic Udeur party from the coalition. He said he favoured snap elections.

"This majority does not exist anymore, this centre left is finished," said Mastella, whose departure with two other Udeur senators wipes out Prodi's two-seat majority in the upper house.

Mastella quit the cabinet after he and his wife came under investigation in a corruption probe. He initially said his party would vote with Prodi, a position he then unexpectedly reversed.

BURNING SHIP