The country's media reported yesterday that the Holy See was unhappy about Laurent Stefanini's nomination for the job in Rome because he's gay.
Officials in Paris have now said they will not change the nominee.
Francois Hollande's government choose Mr Stefanini in January but have still to hear back from the Vatican, widely seen as a rejection.
"We are still waiting for the reply to the request for his validation. Laurent Stefanini is one of our best diplomats, that is why we are proposing him," a source told Reuters.
The French maintaining their position on Mr Stefanini puts the country on a collision course with Rome.
If the Vatican does not accept the ambassador it means France could be left without a senior diplomat at the Vatican.