The Constitutional Court ruled on Thursday that a law banning fired teachers from joining a trade union is constitutional, dealing a blow to a progressive teachers' union.

The Korean Teachers and Education Workers Union, which is currently in danger of being banned by the government, had demanded the law be revised, as dismissed teachers form the majority of its leadership.

Last June, the KTU lost its status as a legal trade union after the Seoul Administrative Court approved the government's decision in 2013 to outlaw it due to its repeated refusal to deny membership to fired teachers.

The law prevents dismissed teachers from being part of a trade union.

The KTU temporarily regained its legal status in September 2014 when the Seoul High Court accepted its request to remain legitimate until its decision.

"Allowing fired teachers to join the union could possibly undermine its independence," the Constitutional Court said, adding that former teachers should not make decisions that could affect current teachers.

Still, the court said it was up to the Seoul High Court to decide whether the union should be banned.

The case had been put on hold at the appellate court pending the Constitutional Court's decision.

Legal experts say Thursday's ruling significantly enhanced chances that the high court will illegalize the union.

The 60,000-strong KTU has among its members dozens of teachers who were sacked for signing statements against the former Lee Myung-bak government in 2009.

Founded in 1998, the progressive teachers' union became a legitimate organization the following year under the liberal government led by President Kim Dae-jung and has grown into the country's second-largest teachers' union. (Yonhap)