A Bulgarian airlines official, jailed here on charges of complicity in the 1981 papal shooting, has been formally advised that he is under investigation for possible involvement in a plot to murder Polish Solidarity leader Lech Walesa, Italian news media reported today.

Italian state television and Italian news agencies Agi and ANSA said magistrate Ferdinando Imposimato sent the notice to Bulgarian Sergei Ivanov Antonov. Similar notices were sent by Imposimato to Mehmet Ali Agca, the convicted Turkish gunman who tried to kill Pope John Paul II in May 1981, and to Luigi Scricciolo, an Italian union official arrested here a year ago on charges of espionage and terrorism, the agencies said.

In Italian legal practice, a judicial advisory is not a charge but a formal procedure that allows a magistrate to interrogate a person on a specific subject. At this stage no indictments have been issued, but sources close to Imposimato say he is working on the hypothesis of a Soviet Bloc spy network operating in Italy and elsewhere with the goal of destabilizing the domestic and international situation.

Agca is serving a life sentence for his role in the papal shooting, and Antonov has been in jail since his arrest in November.

According to ANSA, the first reports of the alleged plot against Walesa came from Agca, who told Ilario Martella, the investigating magistrate in the papal shooting case, that his Bulgarian contacts had asked him if he was willing to try to assassinate the Polish union leader.

Justice Ministry sources have said that Agca was in Rome in January 1981 when Walesa came here to see the pope, and Imposimato, who is conducting the year-old investigation into the espionage charges against Scricciolo, has questioned the Turkish gunman several times in recent weeks.

Scricciolo, formerly the head of the international department of a major Italian union, was arrested in February 1982 on suspicion of seeking to put Bulgarian officials in contact with Italian terrorists and of spying for the Soviet Bloc.

The Italian unionist had close contacts with Solidarity and was in charge of all the organizational details of the Walesa visit to Rome.

No explanation has been given as to why the alleged assassination plan against Walesa--reportedly a powerful bomb to be planted in the Polish leader's car--was never carried out.

The Bulgarian Embassy here has repeatedly proclaimed Antonov's innocence, and today a spokesman for the embassy said, "Any hypothesis that Antonov is involved in such a business is false as is the theory that he had anything to do with the shooting of the pope."