It seemed an innocuous little gift from the French Post Office. ''Come get a Minitel,'' the letter read. Five hundred thousand Frenchmen have responded ''oui'' and picked up the free computers, which were tied into a data bank that gave such practical information as telephone listings and airline schedules.
One journalist, though, decided to play a more serious game with his new gadget. Hooking the machine into a commercial data bank, he gained access to a client's data.
In this case, the client was the French Atomic Energy Commission. The data were plans for the construction of a nuclear reactor and the results of atomic tests in the South Pacific.
When the Canard Enchaine, a satirical weekly newspaper, published the story, the problem of computer theft finally became clear.
Of course, the film ''WarGames,'' along with news reports about American teen-agers who penetrated the computer system of a major hospital, have reached this side of the Atlantic. But experts here say that computer piracy remains marginal here compared with American levels.
'' 'WarGames' seemed an imaginary tale'' to the French, the Canard noted. ''Everyone believed secrets were safe.''
No longer. The Canard's discovery provoked long how-it-was-done demonstrations on television, and a debate about the safety of French state secrets.
The data bank operator charged that the Canard journalist had received the help of ''well-placed individuals.'' But he admitted that some of his clients may not be using all the security measures available.
For its part, the Post Office says it will continue its Christmas promotion of the Minitel. Officials explain that the Minitel is France's hope for the high-tech future. Soon, Frenchmen will be able to use it for home banking and shopping.
The Canard incident has alerted the Atomic Energy Commission. A spokesman said the stolen material contained no dangerous secrets. Such sensitive material , he explained, is kept at the commission headquarters.
But, he added, ''Maybe this will make us a little more prudent.