Four fire engines and around 25 firefighters tackled fire at a suspected cannabis factory on East Lane in Wembley.
Part of a third-floor flat was damaged by fire. There were no reports of any injuries.
A London Fire Brigade spokesperson said: “Cannabis factory fires can be particularly dangerous because of the way criminals use unsafe wiring to illegally obtain electricity in order to grow the plants.
"They're often in top floors or lofts which means when a fire takes hold it spreads, destroying roofs and damaging neighbouring buildings.
"Firefighters can also be exposed to risks when dealing with the fires because of bad wiring and traps at some of the properties.
"It's important that people know the key signs and inform the police so that they can act swiftly to prevent these dangerous fires from happening."
The Brigade’s Control Officers took the first call to the fire at 1539 and mobilised crews from Wembley, Northolt, Park Royal and Willesden fire stations to the scene. The fire was under control by 1838.
The cause of the fire has been recorded as undetermined.