Patrolling police officers who stopped at a Kenton home were amazed to find a "sophisticated cannabis factory" filling five big rooms.
They quickly realised something was suspicious at the Charlton Road address and uncovered 400 ready-to-harvest cannabis plants, in rooms fitted with advanced lighting and air extraction systems.
Police arrested a 27-year-old Albanian man, alleged to be part of an "organised criminal gang", on suspicion of cultivating cannabis, "abstracting electricity" (meaning to dishonestly use, waste or divert electricity) and illegal entry to the UK.
A Met Police spokesman said: "Officers from Harrow Response Team C were on patrol within the Kenton area and attended an address on Charlton Road, Kenton, Harrow, in regards to an unrelated incident.
"Whilst at the location it was evident to officers that there were some suspicious circumstances regarding the address and enquiries were conducted.
"The property had been adapted into a sophisticated cannabis factory, comprising of five large rooms all with cannabis plants. Advanced lighting and extraction systems had been installed throughout.
"This haul would be worth an enormous amount of money to an organised criminal gang."
The man under arrest is currently being detained at a north west London police station while the investigation continues.