The trial of the three Bulgarians and five Turks allegedly involved in the plot to kill Pope John Paul II will not be held until January or February at the earliest, according to senior Italian judicial officials.

The Italian legal system, which has a reputation for being one of the world's slowest, sets a maximum period of 18 months between the formal handing down of indictments and the holding of a trial. In theory, therefore, it is possible that the trial could be delayed until early 1986.

Now that investigating magistrate Ilario Martella has completed his three-year inquiry, his role ends. His report -- together with 25,000 pages of documentation -- will be handed over to the judge of the Rome Court of Assizes appointed to try the case.

The Italian justice system places much greater responsibility on the magistrates appointed to investigate and try the case than the Anglo-Saxon system, according to legal experts here. The judges take the lead in interrogating suspects, weighing the evidence and assessing the guilt or innocence of the accused. Italian magistrates tend to attach more weight than their American colleagues to what is known here as prova logica or the importance of deductive reasoning.

Much can depend on court confrontations between Mehmet Ali Agca and the four Turks and three Bulgarians he has accused of acting as his accomplices.

In Italy, unlike the United States where an accused person is found guilty or not guilty, one of three verdicts is possible: guilty, not guilty for lack of proof, or full acquittal.

By Italian standards, the papal case, involving just eight defendants, has been a relatively simple one. It has not been uncommon in recent terrorist cases for several hundred people to go on trial at once.

The slowness of Italian legal procedures has prompted considerable criticism in recent years. About 70 percent of the inmates of Italian prisons are awaiting trial, according to the president of the justice commission of the Italian Senate.