Ignoring a desperate appeal by Aldo Moro for his life, the Italian government yesterday allowed a deadline set by the former premier's captors to pass without agreeing to the terrorists' demands for a release of leftist prisoners.

The deadline expired at 9 a.m. EST yesterday. There was no indication by late last night of the fate of the 61-year-old Moro, one of Italy's leading political figures and a man who had been considered the country's likely next president.

In a bitter and emotional letter to a leader of his Christian Democratic Party published here yesterday, Moro said that "my blood will be on you, the party and the country" if nothing was done to save his life.

Hours before yesterday's deadline, Pope Paul personally appealed "on my knees" to the "men of the Red Brigades," the terrorist group that seized Moro March 16, urging them "to free Aldo Moro, simply and without conditions . . . because of his dignity as a common brother of mankind."

The Pope's call for mercy followed the publication by La Republica of a secret letter from Moro that reportedly was delivered to Christian Democratic Party leader Benigno Zaccagnini two days ago.

Despite the accusatory tone of Moro's letter, both the ruling Christian Democratic Party, of which Moro is the president, and the Christian Democratic government headed by Premier Giulio Andreotti have refused to consider negotiations with the Red Brigades, who have murdered nine persons so far this year.

On Thursday the terrorists issued an ultimatum in which they gave the government and the Christian Democrats 48 hours to give "a clear and definite answer" regarding the freeing of an unspecified number of "Communist prisoners.

There are more than 300 leftist extremists in Italian jails. Of these, 148 are accused of belonging to the Red Brigades, and 15 leaders of the terrorist organization are on trial in Turin on charges of subversion.

If there was no answer to their demand, the Red Brigades letter said, they would carry out the "death sentence" imposed on Moro a week ago by the terrorists "people's court" that has been trying him.

Since receiving the ultimatum the government has maintained a strict silence. Its position, spokesmen said, is unchanged from April 4 when Andreotti told parliament that the terrorists' "blackmail" was unacceptable and that no deals could be made with persons "whose hands are dripping with blood."

With the exception of the Socialists, who oppose a prisoner exchange but favor other attempts at negotiations, the government's position has the full backing of all the parties, including the Communists, that support it in parliament.

The Christian Democrats have asked Caritas, the international Roman Catholic charity, "to seek a way to convince Moro's kidnapers to free him." But so far Caritas officials in Rome and Freiburg. West Germany, have received no contact from the Red Brigades.

Leaders of both the Christian Democratic and Communist parties reportedly believe that a decision by the government to negotiate with the Red Brigades would have drastic political repercussions here, as well as a demoralizing effect on the Italian police.

Political and intelligence sources here believe that the terrorists may not kill Moro immediately but instead may spell out more precise demands for his release.