The case - brought by two Cuban exiles and a French photographer in the wake of the Pinochet affair - has already caused considerable embarrassment to the French government. The Justice Ministry has made clear it wants nothing to do with the allegations, because President Castro is regarded as a "friend of France".
None the less, substantial prima facie evidence of the involvement of the Cuban regime in cocaine trafficking - including smuggling through the port of Marseilles - has been presented to the examining magistrate in charge of the case.
Judge Herve Stephan, who is also in charge of the inquiry into the death of Diana, Princess of Wales, must decide before the end of this week whether to launch a formal investigation. It is thought unlikely that Judge Stephan will pursue the allegations of crimes against humanity, despite new evidence that Ilich Ramirez Sanchez (Carlos the Jackal) had connections with the Cuban security services at the time of the triple murder he committed in Paris in 1975. Carlos is serving a life sentence in a French jail for these murders.
However, leaks to the French press suggest that Judge Stephan has been taking the possible drugs charge extremely seriously. He has received a video cassette from the United States containing sworn testimony by the former head of the Cuban air traffic control service, as well as a former district attorney in Florida and a former senior US drugs agent.
A former senior official in the Cuban interior ministry, in exile in the US, Antonio Rodriguez Menier, has also written two letters to the judge, in which he alleges that Havana - with the full knowledge of the "maximum leader" - generated funds by assisting cocaine trafficking into Florida in the late 1970s and early 1980s.
Mr Rodriguez Menier also alleges that the Cuban authorities were involved in a cocaine smuggling route through Marseilles in 1979.
This allegation, if taken seriously by Judge Stephan, would give him ample jurisdiction to place President Castro "en examen", the investigative stage of judicial procedure, which is a step short of a formal charge. The public prosecutor has already announced, under government pressure, that he will not be pursuing the allegations against President Castro. But examining magistrates have substantial independent powers to pursue cases once they are entrusted to them.
The complaints were brought by a French photographer, Pierre Golendorf, the exiled Cuban painter, Lazaro Jordana, and by Ileana de la Guardia, the daughter of General Antonio de la Guardia, who was convicted of drug trafficking by the Cuban authorities in 1989 and executed. The trio were inspired by the "success" of the Spanish investigation of the former Chilean dictator, Augusto Pinochet, and especially by General Pinochet's arrest in London pending possible extradition to Spain.
The first two complainants wanted the French authorities to try President Castro for crimes against humanity, both for his repression of dissent inside Cuba and his alleged support of terrorist activities in other countries, including France. Ms de la Guardia brought the separate allegations of drugs trafficking, which seem more likely to succeed.
The affair is an embarrassment to the authorities, which have always maintained friendly relations with Cuba. Danielle Mitterrand, wife of the late president, Francois Mitterrand, is a personal friend of the Cuban leader and remains influential, both with the Gaullist President, Jacques Chirac, and the Socialist Prime Minister, Lionel Jospin.