Corsican pro-autonomy politicians were up in arms on Friday after a court banned the use of the Corsican language in the island's local parliament.
Based on France's constitution, a court in the Corsican city of Bastia ruled on Thursday that French was the only language allowed in the exercise of public office.
Corsican, which is close to standard Italian and has about 150,000 native speakers, is considered by the UN's cultural organisation UNESCO to be in danger of becoming extinct.
Thursday's verdict ruled the Corsican Assembly's custom of allowing Corsican language for debates was unconstitutional and therefore banned, the court said.
Beyond the language question, the court said that local rules effectively establishing "the existence of a Corsican people" were also a violation of the constitution.
The ruling follows a lawsuit brought by the prefect of Corsica, the central government's highest representative on the island, and comes as President Emmanuel Macron's administration is talking with local politicians about granting Corsica greater autonomy.
Leading pro-autonomy politicians immediately lashed out at the verdict.
"This decision amounts to stripping Corsican parliament members of the right to speak their language during debates," said the island's executive council president Gilles Simeoni and Corsican Assembly president Marie-Antoinette Maupertuis.
"Accepting this state of affairs is unthinkable for us," they said in a joint statement, announcing an appeal against the verdict.
The Corsican language needed to be given official status alongside French for it to "survive and develop", they said.
Pro-independence party Core in Fronte tweeted, in Corsican, that it considered the verdict "shameful".
The boss of the "Party of the Corsican Nation", Jean-Christophe Angelini, tweeted that the decision "sounds to us like an insult", also calling it "an injustice and a disgrace".
Corsica has a fraught relationship with France's central government, with nationalist movements having demanded more autonomy or even outright independence for several decades.
Macron said last month that he had "no taboos" about reforming the status of Corsica, which is a sunny Mediterranean island beloved by holidaymakers.
But he insisted that Corsica had to remain part of France.
New negotiations between Paris and Corsican leaders appear to have been unblocked by the conditional release of two men convicted of participating in the 1998 murder of the island's prefect Claude Erignac, the highest-ranking French official to have ever been assassinated.