About 90 firefighters are at a blaze at an industrial estate close to a railway and a major road.
It broke out at about 16:40 BST at a unit in Eastways – between the London to Norwich mainline and the A12 – at Witham.
Essex County Fire and Rescue Service (ECFRS) sent crews from nine stations and said it expected them to be at the scene “well into the evening”.
In an update posted on social media, external, Ben Turner, ECFRS group manager, said: “The fire has affected two premises. We’ve got approximately 90 firefighters dealing with the incident at the moment and it is generating large quantities of smoke.”