Firefighters have been called to a blaze near Wood Lane, in west London, after a fire broke out at industrial building thought to be Unigate Dairy.
By 11am, 15 fire engines and 75 firefighters had been sent to the site, after towering plumes of black smoke billowed across central London.
London Fire Brigade warned that any gas cylinders inside the property could explode and erected a safety cordon in Wood Lane, Shepherd's Bush, west London.
The Metropolitan Police said it had closed the westbound A40 near the area.
A London Fire Brigade spokeswoman said: "There might be some gas cylinders inside the building.
"We have put up a temporary safety cordon as gas cylinders can explode when hot."
More than 100 people have been evacuated from the Unigate factory complex, according to a local petrol station worker.
Rashid Ansari, 23, a sales assistant at nearby Esso in Wood Lane, told the Press Association: "The fire brigade evacuated more than 100 people.
"The fire took place around the back of the Unigate factory. It is a multiple factory so a lot of people work there. It is a welding factory.
"They have closed the road and not a single person is allowed down here. There is lots of thick, dark smoke.
"The fire brigade have controlled the fire right now."
A police spokesman said the A40 was now closed in both directions "so that the fire brigade can work safely".
Eastbound the road was closed from Savoy Circus and westbound from Gloucester Terrace.
Wood Lane was also closed south of Du Cane Road and Shepherd's Bush Lane was closed northbound towards the A40.
Station Manager Rod Horton who is at the scene said: “The fire is affecting a range of industrial buildings on the dairy site. It’s a challenging and developing incident at the moment and we’re likely to be at the scene for some time.”
The Brigade was called at 0935. Fire crews from Hammersmith, North Kensington, Willesden, Kensington, Paddington and Chiswick fire stations are attending the incident. The cause of the fire is not known at this stage.