An Italian court whose task is to corroborate the soundness of criminal charges today rejected a request by defense lawyers for the release from prison of Sergei Ivanov Antonov, a Bulgarian airline official who was jailed four months ago on charges of complicity in the 1981 papal assassination attempt.

Today's ruling by the so-called "freedom tribunal" was the third time since Antonov's arrest Nov. 25 that judicial authorities here have denied appeals for his release from jail, in effect substantiating the evidence on which investigating magistrate Ilario Martella has so far built his case.

The Bulgarian government has insisted repeatedly that Antonov is innocent and that the government in Sofia had nothing to do with the shooting attack on Pope John Paul II.

A spokesman for the Bulgarian Embassy in Rome today denied a report in The New York Times this week quoting a Bulgarian acquaintance of Antonov's as saying that Antonov was afraid of reprisals if he were to return home. The spokesman called charges of Bulgarian and Soviet involvement in the assassination attempt "accusations which collapsed long ago."

The spokesman, Georghe Dimitrov, also denied that Iordan Mantarov, a Bulgarian exile cited in The Times report, was a former commercial attache of the Bulgarian Embassy in Paris. The Times quoted French intelligence officials as saying Mantarov had been told by a Bulgarian intelligence agent that the assassination attempt was engineered by the Soviet Union. Dimitrov said Mantarov, 48, was a maintenance mechanic in the French office of the Bulgarian agricultural firm Agromascinaimpox.

Judicial sources in Rome, however, confirmed that another Bulgarian exile named in the report, Velitchko Peitchev, 52, was brought face to face with Antonov in Rome, apparently in an attempt to convince Antonov to cooperate with Italian investigators.

The Italian news agency ANSA said today's decision by the court, a special tribunal whose function is to corroborate the soundness of criminal charges here, was based on the determination that the evidence presented by Martella, including reported statements by Mehmet Ali Agca, a Turk convicted in the attack on the pope, were sufficient to keep Antonov in jail.

The agency report also said that the court had decided that the testimony by several witnesses who swore that Antonov was in his office at the time the pope was shot in St. Peter's Square was not sufficient to cancel out the charges made by Agca, who reportedly described in detail, even if not perfectly, Antonov's apartment.

However, Giuseppe Consolo, one of Antonov's lawyers, said today that the court's ruling did not take into consideration new evidence that he believes would challenge Agca's credibility because it was based on a petition filed more than six weeks ago.

Consolo said that the new evidence shows that Antonov's wife, Rossitska, left Italy on May 8, 1981, and therefore could not have been present at a meeting on the planned assassination that Agca has said was held in the Antonovs' Rome apartment a few days later. The attack on the pope took place May 13, 1981.

Consolo said that on Monday he would use this information to file a new appeal with Martella for Antonov's release and if that is turned down he will once again file a petition with the "freedom tribunal."