The court declared that strike action was illegal.

法庭宣判罢工为非法。

《牛津词典》

Strictly strike those illegal and fraud activities, and the production and distribution of counterfeited food products.

依法严厉打击各种质量违法和欺诈行为，严格打击食品制假售假等违法行为。

youdao

It is illegal to strike COINS privately.

私造钱币是违法行为。

youdao

Production at the Rustenburg mine was disrupted for eight weeks in 2012 following illegal strike action.

BBC: Amplats plans 6,000 job cuts in South Africa

That might help explain why New York City's transit workers were willing to wage an illegal strike last month.

FORBES: Magazine Article

"Equivalent refined platinum production decreased by 29% owing to the illegal strike action at the Rustenburg, Amandelbult and Union mines, " the company said in a statement.

BBC: Anglo American in platinum production slump