ROME -- Anti-corruption magistrates formally warned Prime Minister Silvio Berlusconi on Tuesday that he was personally under investigation for the alleged bribing of tax authorities, dealing him a blow that could threaten his political survival.


Opposition politicians responded to the warning with calls for the beleaguered prime minister's resignation. But a spokesman for Berlusconi insisted: "The prime minister is not going to resign."


Ansa news agency reported that Milan magistrates had also asked Berlusconi to appear before them for questioning. Warnings like that to the prime minister are normally issued after magistrates acquire enough evidence to suspect a crime and want to probe on a more formal level. They do not imply guilt.


There was no immediate comment from Berlusconi himself, who was in Naples hosting a United Nations conference on organized crime.


Earlier Tuesday, Berlusconi had personally vowed to stay at his post after a report in the Milan newspaper Corriere della Sera that magistrates had placed his name on a list of people who could be investigated for corruption.


"I am serene because, as I have said many times in the past, I am absolutely sure I have not committed any offence," he told reporters in Naples after the publication of the report. He said he was determined to "continue in the service of the state."


His latest refusal to resign was issued by his spokesman, Jas Gawronski, to reporters in Rome.


Media magnate Berlusconi triumphed in March general elections just two months after entering politics.


He was appointed prime minister in May and heads a shaky coalition in which the rightist National Alliance and the federalist Northern League are the main partners of his Forza Italia party.


The formal judicial warning was another hammer blow for Berlusconi after his party suffered a severe setback in the first round of weekend local elections.


The hard-right Communist Refoundation called on Berlusconi to step down while centrist reformer Mario Segni said the warning proved Berlusconi's widely-supposed conflict of interest between his business and political lives would cause trouble for Italy.


The episodes outlined in the report in Corriere della Sera involve suspected bribes paid to tax police in 1991 and 1992 by two companies owned by Berlusconi's Fininvest.


Magistrates suspect executives of the Mondadori publishing company and the Mediolanum insurance group paid bribes in exchange for lenient tax investigations, the newspaper reported.


Several top officials of the elite finance police, which carries out audits, have been arrested on charges of demanding bribes from companies.