ROME — Italian President Giorgio Napolitano warned political factions on Thursday against endangering Prime Minister Enrico Letta's fragile coalition government, which is facing its most serious threats since being formed three months ago.

Explicitly mentioning the legal problems of former premier Silvio Berlusconi and uproar over the hurried deportation of the family of a dissident Kazakh oligarch, Napolitano said the stability of the government must not be called into question.

"The damaging effects on our international relations and on financial markets would be seen immediately and could be impossible to recover from," he said in a speech to a journalists' association.

Napolitano's speech came with Letta's uneasy coalition of traditional rivals on the left and right under increasing strain over issues ranging from Berlusconi's future to tax and economic policy.

As well as a July 30 verdict on Berlusconi's final appeal against a tax fraud conviction, which could see the billionaire media tycoon barred from public office, the government will confront a vital test on Friday over the future of Interior Minister Angelino Alfano.

Alfano, the secretary of Berlusconi's center-right PDL party and deputy prime minister, faces a no-confidence vote over the deportation of the family of dissident Kazakh oligarch Mukhtar Ablyazov in May. The head of his office in the interior ministry has already resigned over the scandal.

The government has rescinded the deportation order, although the family is now in Kazakhstan, and ordered a shake-up of the various public offices involved after acknowledging severe failures in the way the case was handled.

Letta is firmly backing Alfano but a group within the prime minister's center-left Democratic Party has urged a vote against the minister, despite warnings from the PDL that the government will collapse immediately if he is forced to resign.

Napolitano made no direct comment on the no-confidence vote but was severely critical of the way the "inconceivable" case had been handled by officials and of the role of the Kazakh authorities in pressuring Italian authorities into the deportation.

The strength of the combined government majority in parliament should ensure Alfano survives the vote, unless a large body of PD parliamentarians break ranks, but the case has shone a harsh spotlight on the tensions within the government.

Letta's administration is also awaiting the unpredictable outcome of the July 30 verdict in which Italy's top appeals court will decide whether or not to uphold a four-year prison sentence and five-year ban on holding public office against Berlusconi.