More than 1,000 cannabis plants with a potential street value of £1.1 million were seized during a raid in Kingswood.
Officers from Avon and Somerset Police descended on the property thanks to a tip-off from a member of the public.
Inside, detectives discovered electricity had been diverted from a nearby substation to power the equipment used to grow the plants. Western Power Distribution have since made the building safe.
Specialist crime scene investigators are now searching the venue for evidence and all of the cannabis and equipment have been removed by the force's Operation Viscount team.
Drugs seized during the raid will now be destroyed.
PC Henry Bird said: “Those responsible for producing these plants were not doing it as a hobby or for their own personal use, they are organised criminals who we believe were exploiting vulnerable people to unscrupulously line their pockets.
“This seizure is a significant blow to these criminals and has prevented a large amount of drugs from hitting our streets and having a detrimental effect on our communities.”
He added: “Not only is there often a very human cost to organised cannabis grows such as this but they also pose a very real risk. Abstracting electricity in the way done at this unit is obviously illegal but it’s also extremely dangerous. It can be a real fire hazard.
“It’s thanks to the information provided to us by a member of the community we were able to take the action we did and I hope this reassures people we will always listen to you.
“Please keep coming to us with information, it really does make a difference to disrupting organised crime, safeguarding vulnerable people often trafficked or kept as slaves and to preventing the sale of illegal drugs in our communities.”
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