A large-scale cannabis plantation was discovered by an elderly landlord at a house he rented out three months ago.
Drug farmers had stripped out all the doors and furniture of the newly refurbished home in South Acton and set up lighting and irrigation equipment to service dozens of plants in several rooms.
Neighbours initially raised the alarm after noticing a smoking electrical cable feeding into the house in Princes Avenue on Tuesday. They watched the tenants frantically packing plastic bags full of the drug into a white transit van parked outside.
By the time the 86-year-old owner, who had been investigating an electrical overload, and police arrived, the criminals had fled. Officers have been examining the house and dismantling equipment since.
"It was unexpected and most surprising," said the landlord, who asked not to be named.
"You have to see it to believe it. There were ducts for ventilation and ultraviolet lamps to provide light and heat.
"By the time I got there they had taken what they could and left.
"It was a fully furnished house, and now all the doors and furniture have been taken out so they had more space to do their work."
The criminals, who took over the house three months ago, paid the rent by cheque and had not been seen by the landlord since they first moved in. They have been unreachable since the discovery of the drugs factory.
Neighbour Carl Levesconte said: "They knew what they were doing - it was a massive operation.
Every room was full of equipment and they'd knocked holes in the walls.
"They came down with a big van and were going up and down the garden with big bags for at least an hour."