Lancashire police discovered a £250,000 cannabis farm in a former church building in a raid on Sunday evening.
Having been alerted to a disturbance by a member of the public, officers went to Burnley Lane Baptist Church in Burnley to find around 600 cannabis plants.
The church was sold and repurposed in 2008; the current landlord said he was "shocked to the core" to find out about his tenants' line of work.
"I rented that part of the building around four months ago to some guys who wanted to set up a computer business," the landlord told the Lancashire Echo.
"Recently they changed the locks, which made me suspicious, but they continued to pay their rent.
"They seemed like ordinary, respectable people," he said.
The farm was found in a part of the building that used to be used for Sunday school, and not the main church hall.
The Baptist church congregation vacated the building when they joined with a local Methodist church.
Inspectors spent much of Monday collecting evidence from the scene, including plants, lamps and tin foil.
No arrests have yet been made.
