European Union leaders have slammed the decision, which comes at a delicate time during Turkey’s bid for EU membership.

News of the ban’s reinstatement on May 25 stunned teachers and rights activists. The prohibition is over the union’s support for mother-tongue education, which it has enshrined in its constitution.

It backs teaching in Kurdish, the mother tongue of an estimated 12 million Turkish citizens. Conflict between ethnic Kurds and Turks recently surged in the country’s Kurd-dominated South-east.

The union’s stance led to a court case last year which ruled that teaching in languages other than Turkish undermined the country’s constitution. The union, Egitim Sen, was served with a ban unless it abandoned support for the practice. The decision was overturned by a higher court, but now that ruling had been reversed by the still higher supreme court, once again banning it .

“There is serious pressure on the right to be organised,” said union leader Alaaddin Dincer. “This is a democracy with a mask.”

The decision has also added to a growing concern over Turkey in the EU. The country is due to start accession talks in October, thanks to a good recent track record of democratic reforms. These had included introducing Kurdish language TV broadcasts and allowing Kurdish language schools. The EU told Turkey last week that the ban was evidence of backsliding on these commitments. Turkish education officials have declined to comment.

With 210,000 members Egitim Sen is Turkey’s biggest teachers’ organisation.

Its protest over the ban has included demonstrations outside parliament. It has a final chance to stop the ban being implemented - an appeal to the Supreme Court of Appeals, which will rule later in the summer or early autumn. But as the Supreme Court’s decision was unanimous, few think it will succeed.

“We have to protest now and make them take back their decision,” says Omer Karikoglu, a high-school teacher and union member in Istanbul’s Besiktas district.