The drugs factory is one of the largest ever found by police in the West Midlands and is believed to have been in operation at the old Kenrick building next to the M5 in West Bromwich for six months.
Thousands of cannabis plants were found growing across all three floors of the Grade II-listed Victorian building as well as in the loft space and the clock tower, while extensive heating, lighting and hydroponics had been installed. People are also thought to have been living inside.
WATCH: Take a tour of huge cannabis farm
The farm was busted last month when a utilities company became suspicious about a surge in electricity use.
Nobody has been arrested but detectives are celebrating a "big win" in the fight against organised crime.
West Midlands Police Detective Inspector Ade George, said: “We strongly suspect this drugs factory, given the scale, is linked to serious organised crime.
“It’s a big win for us and its discovery, especially as the crop was nearing harvest, will be a significant dent in the finances of the crime group involved.
“We’ve seized several million pounds worth of drugs plus equipment also costing tens of thousands of pounds.
“A drugs factory of this size, with all the modifications, takes a lot of time and effort; I would suggest it has been in operation for around six months. There is also clear evidence of living quarters within the property.
“No-one has been arrested at this stage but our enquiries into who was behind the factory are underway - and I’d urge anyone with information to get in contact."
The farm was concealed from view at Haliburton House in Hall Street South, also known as the Kenrick building or Archibald Kenrick House.
The building housed metal producer Archibald Kenrick & Sons and dates back to the late 19th century but has been out of use for more than 20 years. It made the news during the general election campaign last year when candidate George Galloway draped a 42-foot-wide banner on the side of the building in view of passing motorists on the M5.
The cannabis plants inside were close to maturity when police raided the building on July 14 and are estimated to be worth up to £3 million. They have since been removed for destruction, with specialist contractors called in to help due to the asbestos risk.
Chemical containers, water butts, hosepipes, fertiliser, sprayers, electric water pumps, duct pipes and masses of wiring, fuse boards and power banks were also found in the building.
Inspector Scott Shaw, manager of the Cannabis Disposal Team, said: “The construction of the cannabis farm disturbed asbestos construction material in the building; both the plants and property were contaminated as a result.
“We worked with the Health and Safety Executive and specialist contractors to clear the site and make it safe.
“The health hazards for anyone working in this environment are very clear − but there would also have been a serious danger to anyone using asbestos contaminated drugs.
“Discovering this drugs factory will have had struck a significant financial blow to the organised criminals behind it but it’s also protected members of our communities from very serious harm."
Plans to turn the building into flats were refused by Sandwell Council last year and dismissed again on appeal in March.
Anyone with information or who has seen suspicious activity around the building in recent months - or may have dashcam or cyclecam video - is asked to contact police quoting crime reference number 20SW/169506P/20.