Representatives of Brittany’s Diwan schools, which teach in Breton, have reacted with anger and dismay following a ruling by France’s Council of State that their teaching methods were unconstitutional.

The council barred the schools from becoming part of the state education system.

The decision is a blow to education minister Jack Lang’s plans to integrate the schools into the public service, a cornerstone of his policy to promote regional and minority languages. The council agreed with two applications lodged by secondary teachers’union, SNES, and the CNAL (Comite National d’Action Laique), a federation of educational unions and the biggest parents’ association that supports state education.

These held that the immersion method used by the 35 Diwan schools, where pupils are taught all subjects in Breton, infringed the principles of equality and unity of the Republic, where constitutionally French is the only national language. SNES and CNAL said they were not against regional language teaching if French was given parity.

Lang announced plans last April to set the teaching of regional and minority languages on an equal footing with foreign languages, and signed an agreement to grant Diwan schools public status in May. This was expected to pave the way for other regions to develop teaching of languages such as Corsican and Basque.

Diwan supporters reacted angrily to the ruling. The Union Democratique Bretonne de-nounced the “Taliban of Jacobinism who set themselves up as guardians of supreme truth”, and called for public demonstrations.

Others condemned a “totally obsolete constitution” and “an inadmissible decision” which “yielded to a litigious minority fossilised in the centralism of another century”.

Lang reportedly remained determined to continue the policy. In a statement to Le Monde, he said if there were a legal obstacle he would overcome it.