National Crime Agency officers were alerted to a former nightclub in Coventry, Warwickshire on Thursday.
Officers smashed through several reinforced doors to access the huge grow, which spanned across three floors in the huge building.
NCA officials have valued the grow of around 1,000 cannabis plants at more than £1m.
Sophisticated planting, growing and irrigation equipment, which has been valued at around £150,000, was also recovered.
Three men were arrested in connection with the grow – two of which had to be tracked down by drone as they fled the building.
NCA Branch Commander Adam Warnock said: “This is a significant drug seizure which will have a huge impact on the organised crime groups involved, depriving them of commodity and profit.
“The size of the cannabis factory…