Twelve fire engines and around 80 firefighters are in attendance at a fire at a recycling centre on Springwell Lane in Harefield.
Around 80 tonnes of scrap metal is alight.
The fire is producing a large amount of smoke and people in the local area are advised to keep their windows and doors closed at this time due to smoke travel.
Springwell Lane is currently closed.
Firefighters are expected to remain on scene throughout the day as they work to extinguish the fire.
The Brigade was first notified about this incident at 0941 and Control Officers have mobilised crews from Ruislip, Harrow, Hillingdon, Stanmore, Northolt and Barnet fire stations to the scene. Several fire crews are also in attendance from Hertfordshire Fire & Rescue Service, alongside an appliance from Buckinghamshire Fire & Rescue Service.
The Brigade's drone team are also in attendance, providing the Incident Commander with a greater situational awareness of the fire.
The cause of the fire is unknown at this time.