The blaze broke out at Tyseley Industrial Estate in Birmingham on Monday afternoon.
The fire is believed to have involved plastics recycling company GW Webb Plastics.
West Midlands Fire Service said the "severe fire" would last late into the night last night and urged people to avoid the area.
The Hazardous Area Response Team from West Midlands Ambulance Service was sent to the scene along with police officers but nobody is thought to have been injured.
Thick clouds of grey and black smoke were pictured rising into the sky by dozens of concerned onlookers after the fire broke out.
Nearby residents were advised to keep their doors and windows closed to avoid toxic fumes from the fire, with smoke seen as far as Oldbury, 10 miles from the scene.
Part of the A41 Warwick Road was closed as a result, with heavy congestion in the nearby area.
A West Midlands Fire Service spokesman said: "We now have more than 100 firefighters tackling a severe fire at a plastics factory on Tyseley Industrial Estate, Birmingham.
"This incident will last into the night. Please avoid the area and keep windows and doors closed if you live nearby."
Footage of the blaze:
Our @WMASHART drone has been sent up over the fire to gain perspective on what our @WestMidsFire colleagues are dealing with. Our staff along with @OFFICIALWMAS senior paramedic officers remain on scene to support firefighters and act as a link for @DHSCgovuk pic.twitter.com/J4s5OvLOPE— ☤ WMAS HART ☤ (@WMASHART) August 10, 2020
We now have more than 100 firefighters tackling a severe fire at a plastics factory on Tyseley Industrial Estate, Birmingham. This incident will last into the night. Please avoid the area and keep windows and doors closed if you live nearby. pic.twitter.com/13wYOdhT2w pic.twitter.com/sHtX9kPlkQ— Perry Barr Fire Station (@WMFSPerryBarr) August 10, 2020