Romano Prodi, the Italian Prime Minister, has called a vote of confidence in Parliament tomorrow that could bring down his fragile coalition after the defection of a key Christian Democratic party.

If Mr Prodi falls, President Napolitano can call new elections or appoint a caretaker government to introduce electoral reform in a bid to avoid future instability. This has emerged as a plausible scenario, with Pierferdinando Casini, an opposition leader, calling for "a government of national responsibility". Silvio Berlusconi, the Centre Right leader, insisted that he wanted elections in the spring "which we will win".

The crisis unfolded when Clemente Mastella, the Justice Minister, offered his resignation last week after both he and his wife Sandra were implicated in a corruption inquiry. Mr Prodi at first refused to accept the resignation, but Mr Mastella insisted that he could not carry on. "If there is a vote of confidence we will vote against the Government. This Centre-Left government is finished," Mr Mastella said. "There comes a time when a man says enough is enough."

In a speech to Parliament this morning Mr Prodi rejected Mr Mastella's allegation that he had failed to back him over the corruption allegations. "Mastella was not left on his own," Mr Prodi told deputies, adding that he could not interfere because the independence of the judiciary had to be respected.

Mr Prodi said the fate of governments was decided by Parliament, "not by news agencies or television debates". He expected to win the confidence vote because "this Government was born with an electoral programme signed by all parties". The Government was "putting Italy back on its feet", had "reacquired credibility abroad", had tackled tax evasion and put liberalisation top of its agenda. Mr Berlusconi said that the fall of the Prodi Government was "inevitable".

Mr Mastella, noted for his volatile temperament, initially he said that his UDEUR party would support Mr Prodi from outside the coalition case by case, but then on Monday night changed his mind and said it would vote against Mr Prodi in a confidence vote. The UDEUR has three seats in the Senate, where Mr Prodi has a majority of only one. He has a comfortable majority in the Lower House.

In the 18 months since being elected Mr Prodi has several times had to rely on the votes of seven unelected life senators for a Senate majority. Coincidentally, the Environment Minister, Alfonso Pecoraro Scanio, also faces a vote of confidence in the Senate tomorrow over his handling of the Naples rubbish crisis.

Mr Mastella said Mr Prodi's coalition was "finished" and that he favoured new elections. He said that he had phoned Mr Berlusconi, who, according to opinion polls, is likely to be swept back into office if elections are held despite divisions in his Centre-Right coalition.

President Napolitano has repeatedly said that he wants new election rules before permitting an election because the system of proportional representation awards parliamentary seats to even very small parties, giving rise to unstable multiparty coalitions.

The Prodi Government has governed with a fragile majority since narrowly winning the closest elections in modern Italian history in April 2006. The 2008 budget was approved last month with the 31st confidence vote in the Prodi coalition.

Because of economic decline and industrial unrest the Government's popularity has plummeted, with polls showing that barely 30 per cent of Italians have "confidence" in the Prime Minister, down from 45 per cent a year ago.

The political turmoil also affected Italian financial markets, with Alitalia shares plunging over concerns that the collapse of the Prodi Government could derail the planned sale of the troubled airline to Air France-KLM.

The Centre Right has also been in disarray since losing the 2006 election. Last November Mr Berlusconi's two main allies, Gianfranco Fini, head of the Far Right Alleanza Nazionale, and Mr Casini, a Christian Democrat, scathingly dismissed an announcement by the Forza Italia leader that he was launching a new movement called The People of Liberty as a "meaningless spectacle".

Today Mr Fini insisted that the Centre Right would enter new elections as a united force, with Mr Berlusconi at its head.

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