Emergency services were called to the factory at approximately 12.50pm yesterday (November 23).
Police said the blaze originated in the cooling system and quickly spread. There were no reports of injuries.
The Belgian capital, Brussels had to be closed off, creating traffic chaos as the evening rush hour approached and Eurostar trains were suspended momentarily as the fire was tackled.
Pierre Meys, spokesman, Brussels fire service, said there are no reported casualties as a result of the fire and the cause of the blaze is not yet known.
Brussels mayor Marc-Jean Ghyssels warned the smoke could be toxic and urged residents to keep their windows closed.
"A significant amount of smoke is visible and we have asked the population to close the doors and windows of their homes, and to stay inside," said Ghyseels.
Up to 50 firefighters have contained the blaze, reports say, but thick black smoke could be seen bellowing from the factory from miles away.
A security perimeter has been set up around the factory and a nearby school of about 200 children and neighboring businesses was evacuated.
Incendie Milcamps Forest pic.twitter.com/ptcMkFPBf8— Cédric De Cock (@CedricDeCock) November 23, 2017