A secret tunnel has been found leading to an underground cannabis farm in the basement of a former Victorian theatre.
The 70ft passage ran from the toilet of a four-bedroom terraced house into a vast space below a row of shops which replaced the theatre in the Sixties.
Detectives believe the illegal factory may have been operating for about a decade, raking in more than £4 million each year, a source said.
A criminal gang is understood to have tapped into the main power supply via underground cables in order to heat the cannabis plants.
Police found the tunnel after neighbours reported an attempted break-in at the house in Deptford, south-east London, and officers noticed that the toilet floor was hollow.
In scenes reminiscent of Second World War film The Great Escape, the gang had tunnelled down from the ground-floor toilet, building a trap door covered with tiles to hide the secret passageway.
The gang had then dug horizontally underneath a private car park behind the house before emerging into the large, abandoned basement.
A second vertical shaft was built up to the car park for ventilation and as a possible escape route.
In scenes reminiscent of Second World War film The Great Escape, the gang had tunnelled down from the ground-floor toilet.
The vast space, which cannot be accessed by the shops above, is the remnants of the 750-seater Broadway Theatre, which was built in 1897. Demolished in 1963, the deep foundations were left behind and the area was redeveloped.
The £430,000 property where the tunnel began has been boarded up. Police believe at least one gang member was a builder because of the tunnel’s skillful design.
A businessman who lived next door to the house said: ‘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.’
The businessman said two men had broken into the back of the house in August and stolen a set of keys. But they had gone to the wrong front door and, when he tried to confront them, one tried to fight him while the other ran away.
In September, he said two men had again targeted the wrong house and had kicked down the door of a neighbouring home which was occupied by students.
On October 1, police were called a third time and this time they found the tunnel.
It is unusual to have such a large-scale cannabis operation secretly running next to a busy high street.
Earlier this year, police found hundreds of cannabis plants in an underground bunker filled with lights, fans and heaters in a back garden in Wolverhampton.
And, in 2017, three men were jailed for running an underground cannabis farm in a former nuclear bunker in Wiltshire. Staffed by young Vietnamese workers in slave-like conditions, it was capable of producing £2 million of cannabis a year.
A spokesman for the police 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.’