Firefighters are tackling a blaze involving 160,000 litres of cooking oil at a recycling centre.
They were alerted to the fire in Little Warley, near Brentwood in Essex, at 5.15am on Tuesday.
Fifteen appliances are dealing with the blaze, with fire crews from London drafted in to provide support, which also involves 30,000 tonnes of paper and 30,000 litres of diesel.
Utility companies and the Environment Agency have been called in by the fire service.
Incident commander, divisional officer Mark Samuels, said: "Firefighters have been aggressively fighting the fire with foam. This is a recycling yard that recycles various types of used oils, hence the intensity of the fire.
"We are in the final stages of tackling the blaze, although we expect to be on the scene for several hours."
A fire service spokesman added: "Once the fire is out, a joint police and fire service investigation will be carried out to establish the cause."