A bid to inscribe Breton and other regional languages in the French constitution has been thwarted after the Académie française - the elite body that guards over French culture - ruled that it was a threat to national unity.

Under a bill to update the 1958 Fifth Republic constitution that is currently going through parliament in Paris, deputies voted last month for an amendment to include in the preamble the words: "Regional languages are part of France's heritage."

It would be the first time that Breton, Catalan, Basque, Occitan, Corsican and other minority tongues received official status, after centuries in which they have been at best neglected and at worst actively suppressed.

But the prospect aroused the wrath of the "Immortals" - as the Académie's members are known - who woke from their normal somnolence to issue a ringing denunciation.

While paying tribute to the role of regional languages in "enriching" French culture, the sages argued that to elevate them to the constitution would have "grave consequences".

"Above all, it would cast into doubt the principal of equal access for all to the administration and justice," they said.

Noting that the proposed clause would actually precede Article 2 of the constitution - which states that "The language of the Republic is French" - the Académie said that "to place France's regional languages before the language of the Republic is a challenge to simple logic and a denial of the Republic."

Founded by Cardinal Richelieu in 1635, the Académie Française comprises 40 pre-eminent writers and academics - many now in their dotage - whose principal task is to write the definitive French dictionary. It is a Sisyphean task - when one edition is finished after many years, they begin on the next - and they are currently at the letter P of the ninth.

Immediately after the Academy's statement was released last week, the proposed constitutional amendment was easily overturned by a vote in the upper house of parliament - the senate - which is dominated by conservative figures of both the right and the left.

Among those arguing against the change was the Socialist Michel Charasse, who asked why, of all France's cultural heritage, only its regional languages deserved special treatment. "Why don't we inscribe our historical monuments too? Or our food? Why not Auvergnat hotpot?"

Jean-Pierre Fourcade, a senator from President Nicolas Sarkozy's UMP party, said: "Today our children talk in telephone text. What we should be doing is defending French, not seeking help from the regional tongues."

The row is the latest confrontation in the long-running culture war that has pitted centralisers in Paris - pejoratively dubbed Jacobins after the hardliners of the revolution - against defenders of regional identities.

For most of the last two centuries, national governments have done their best to stamp put local peculiarities - largely out of an atavistic fear that without strong central command the country would split into its original constituent parts.

As far as languages were concerned, this meant sending out teams of teachers to ensure that children from Nice to Lille spoke approved French. In Brittany, there were signs in classrooms that read: "No spitting. No speaking Breton."

In the past 30 years there have been major strides in favour of regional languages and today there are schools that teach in Breton (diwans), Occitan (calandretas), Basque (ikastolas) and Catalan (bressolas). Statistics are unreliable, but there are probably some 300,000 people who can speak Breton.

Nonetheless, France has refused to ratify the European Charter for Regional and Minority Languages, on the grounds that it is against the 1958 constitution, and today Breton is the only living Celtic language that does not have official recognition.

Needless to say, regional campaigners are furious at the setback and are urging the government to insert a modified version of the language clause when the bill comes back for a second reading in the National Assembly.

"This rejection by the Senate may not be the death blow to our regional languages, but it extinguishes the hope born just a few weeks ago that France would go down the democratic road of plurality and cultural and lingustic diversity," said Mona Bras, of the pro-autonomy party the Breton Democratic Union (UDB) "French laws will have to be brought up to date with what is happening everywhere else in Europe," said David Grosclaude of the Institut d'Estudis Occitans which promotes Occitan, the language of southern France.

But the establishment newspaper Le Monde was sceptical, asking whether regional languages need constitutional protection to continue their renaissance. It drew a link with the film Bienvenue Chez Les Ch'tis - set in the Ch'ti dialect-speaking north - which has been seen by 21 million people.

"This film has done more for the regional language of the north than any legal text. With humour, and without a national drama," it said.