Italian Prime Minister Romano Prodi's tenuous grip on power weakened further today after a cabinet minister, resigning over a graft probe, said his party had left the government and would only offer "external" support.

Clemente Mastella, whose Catholic party has been crucial for Prodi's centre-left coalition to secure a majority since coming to power in 2006, quit as justice minister yesterday after hearing that he and his wife were being investigated.

While Mastella's support had often wavered anyway and he fought with leftist allies and even threatened to bring down the government, the loss of his small Udeur's three Senate seats would hurt Prodi, who has a majority of only two seats there.

"We will give external support ... And we will be demanding and not accepting compromises as we have until now," 60-year-old Mastella said, adding that Udeur leaders would next month debate their future relationship with the ruling coalition.

"Today we are not in the government; later we will see," he told a news conference in his home region of Campania, around Naples, where his wife Sandra chairs the regional council.

Prodi told parliament he hoped Mastella would resume his job once the court proceedings are over and was taking over the ministry himself in the interim. Prodi seemed confident he would continue to have Mastella's backing in parliament, saying his government "has counted on his party's support in the past and will count on it in future".

But the centre-right opposition said Prodi should quit. "Prime minister, your government's time is up!" said Elio Vito, a deputy in Silvio Berlusconi's Forza Italia party.