ArcelorMittal has told unions that it is permanently closing its two blast furnaces near the eastern Belgian city of Liege.
One of those furnaces has not been producing steel since 2008, the other since August. A foundry will also be shut.
The world’s number one steelmaker partly blamed weakening demand due to the worsening economic situation.
As many as 1,000 workers and contractors will lose their jobs.
CSC union head David Camerini said: “It’s a tragic situation. It’s a real disaster for the steel works at Liege and particularly for all workers and their families. But we’re not going to give up, we are going to look at all the possible alternatives to keep the steel industry in Liege.”
Workers at the plant in Liege recently prevented six company executives from leaving their offices for 48 hours in protest over temporary closures and layoffs.
Local media said the closures will also indirectly effect thousands more workers in the area.