Fifteen fire engines and around 100 firefighters were called to a fire near Roxeth Green Avenue in South Harrow overnight.
The fire occurred at a range of four railway arches, housing businesses including vehicle workshops. The units have been severely damaged by fire.
Firefighters have worked hard to bring the fire under control but the Brigade is likely to maintain a significance presence on scene throughout the day. There have been no reports of any injuries at this time.
Station Commander Graham Beers, who was at the scene, said: "Crews were faced with a substantial fire affecting multiple commercial units situated within a railway arches. Whilst most of the fire has now been extinguished, firefighters will remain on scene through today putting out deep-seated pockets of fire, dampening down hotspots and ensuring the scene is safe.
"We asked people in the local area to keep their windows and doors closed during the height of the fire due to smoke travel. Around 300 people were also temporarily evacuated from nearby homes as a precaution due to the proximity of the incident and smoke. Road closures remain in place in Roxeth Green Avenue, near the junction of The Arches, and are expected to remain in place whilst we continue to work at the scene."
The Piccadilly Underground line, which runs over the railway arches, has been severely impacted by this incident. Transport for London has confirmed the Piccadilly line is currently suspended between South Harrow and Uxbridge stations.
During the fire, an unknown number of gas cylinders at the site are believed to have exploded during the early stages of the incident. Firefighter safely removed two further cylinders and cooled them to an ambient temperature, as gas cylinders can explode when exposed to heat.
The Brigade received the first of around 40 calls about the fire at 0118 and mobilised crews from Northolt, Harrow, Wembley, Stanmore, Ruislip and surrounding fire stations to the scene. Two 32-metre turntable ladders have also attended and been used as water towers to help extinguish the fire from above. A drone has also been deployed, providing the Incident Commander with a greater situational awareness of the incident.
The fire was brought under control by 0401.
Speaking on the morning after the fire Assistant Commissioner Jamie Jenkins said: “Firefighters remain on scene at the site of last night's fire at a set of railway arches near Roxeth Green Avenue in South Harrow.
“Crews are working closely with our partners at TfL to assess the damage caused by the fire to the railway arches, as well as carrying out operations to make the scene safe. As part of this, the Brigade’s Drone is being deployed to the scene to carry out an aerial survey of the area. With TfL, we will continue to work to safely re-establish service along the section of the London transport network impacted by this fire."
The cause of the fire is under investigation.