Police estimate the cannabis produced at the old 750-seater Broadway Theatre, which was converted into shops in the 1960s, could have surpassed £40 million in the past decade. They found the farm when responding to reports of a break-in at a £430,000 four-bedroom terraced house nearby.

An officer noticed the floor in the toilet was hollow and after further inspection, they found a tunnel had been made from the bathroom leading to a private car park and towards the old theatre’s large basement.

The criminal gang, who also built a second shaft for ventilation, had tried to hide the entrance in the toilet with a secret trap door. They chose the theatre’s basement because it can no longer be accessed by the public.

A man who lived next door told Mail Online: “To think this has been happening next to us for ten years without any of us noticing is certainly surprising. I was shocked.

“My wife and I sometimes heard what seemed like drilling. It was never for very long but it vibrated the walls upstairs.

“We had no idea what it was, we just thought it was some building work nearby – it’s a busy road.”

A Met Police spokesman said: “On October 1, officers were called to an address following reports of a disturbance. They discovered a large number of cannabis plants along with equipment used in the cultivation of cannabis in an area beneath the residential properties.

“Three men, aged 28, 45 and 47, and a 36-year-old woman have been arrested on suspicion of the cultivation of cannabis. They have all been released under investigation.”

Police have boarded up the home as they continue to investigate.