Mr Prodi is being forced to "reaffirm" his foreign policy

The BBC's Christian Fraser in Rome says Mr Prodi often gets more backing from the opposition than his own alliance.

Last week three ministers boycotted a cabinet vote on funding to keep Italian forces in Afghanistan.

And Christian Democrats say they will destroy the government if it gives full rights to unmarried and gay couples.

Our correspondent says the coalition appears to be tearing itself apart.

Intervention

The rift in the coalition has forced Mr Prodi to call a meeting next week to "reaffirm", as he puts it, the government's foreign policy, particularly when it comes to its peacekeeping missions.

This week Mr Prodi won a vote on a controversial plan to allow the Americans to enlarge a military base in Vicenza. It passed by a six-vote majority.

A vote on the US base in Vicenza revealed the coalition cracks

But closer scrutiny of the vote reveals just how fragile is Mr Prodi's grip on power, our correspondent says.

Six of his senators were absent, drawing attention once again to the fact that on important foreign policy issues the government gets more support from Silvio Berlusconi's opposition than it does it from its own centre-left allies.

The crisis has worsened to the point where Italy's Deputy Prime Minister, Francesco Rutelli, was forced to intervene, accusing left-wingers of going too far with their internal opposition to foreign and defence policies.

Mr Rutelli told journalists the government would not retreat an inch from its close ties with the US, despite last week's cabinet vote walkout by two communist ministers and a Green minister.

But according to our correspondent, it is on matters closer to home from which Mr Prodi faces a bigger threat.

From the centre of his coalition, a small party of Catholic Christian Democrats, headed by the Justice Minister Clemente Mastella, says it will oppose the forthcoming bill to give full rights to gay and unwed couples.

Mr Mastella says he would rather see the government fall than vote in favour of it.