About 70 firefighters were battling a blaze on an industrial estate in Watford in the early hours of July 13.
Smoke and fire could be seen from a great distance in a building on Rhodes Way, off Colonial Way, close to Watford Junction Station.
Hertfordshire Fire & Rescue said that at least 37 calls had been made and a number of special resources were required to tackle the fire, which is understood to now be under control.
Hertfordshire Police were also at the scene and were warning people to avoid the area and keep windows and doors locked.
Herts Fire Control tweeted: “We are currently dealing with a commercial building fire on the Reeds Estate in Watford. We currently have six appliances and various special resources in attendance. 37 calls so far and they are still coming as the fire can be seen from some distance away.”
Picture courtesy of @Javed_Laher