German prosecutors have been investigating Volkswagen since 2015 after it emerged that the car giant had installed devices in 11 million diesel cars designed to cheat pollution tests.
French prosecutors opened their own probe two years ago into alleged aggravated fraud at Volkswagen.
"We have already sent three official requests for documents to the prosecutor of the court in Braunschweig, requests which have come to nothing," three French investigating judges wrote in a letter dated 27 July.
German prosecutors "consider that the communication of elements of their inquiry to French judges would seriously risk disrupting the progress of their investigations", the judges wrote to car-owners who are civil plaintiffs in the case.
Volkswagen is being treated as an "assisted witness" in the French investigation -- halfway between being charged and serving as a witness.
Billions in compensation and fines
German prosecutors have opened probes into alleged fraud, stock price manipulation and false advertising against Volkswagen and its Audi and Porsche brands, as well as at Daimler and electronics maker Bosch.
In France, three other carmakers are the target of similar investigations: Renault and PSA, as well as US-Italian group Fiat Chrysler.
Earlier this month Volkswagen reported a leap in second quarter profit thanks to strong sales, even after it had to shell out another 1.6 billion euros in costs related to the scandal.
Dieselgate has cost it over 27 billion euros so far in compensation, buy-backs and fines.