Firefighters have set up a hazard zone after a fire in a workshop.
Crews from Colchester were called to Lodge Farm, on Boxted Road, Great Horkesley, at 9.38am.
The blaze was put out by 10am.
But a 100 metre evacuation zone remains as an oxy-acetylene cylinder was in the barn which caught fire.
Once it reaches a critical temperature the cylinder could explode at any time until is called down.
Fire crews remain on scene cooling down the cylinder.
A spokesman for Essex County Fire and Rescue Service said the exclusion zone could remain in place for several hours.
It is not yet known how many homes have been evacuated but the incident is in a rural area.