Police discovered 20 hidden rooms inside the former mill, containing 1,300 cannabis plants valued at about £760,000.
It was found by police officers who smashed their way through breeze block walls inside the building in Darwen, Lancashire.But the stash was completely hidden from plain sight – with a lift providing access to upper floors of the building disguised behind a stone fire place.
A 31-year-old man was arrested inside the mill, which had specially built living quarters designed for long-term use, police said.
Detective inspector Vinny De Curtis said: ‘Almost every single room had been converted for the wholesale production of cannabis plants.
‘This was an enormous set-up – I am convinced it is one of the most sophisticated set-ups in the UK.’
It is believed the building was used by organised crime gangs to create industrial amounts of cannabis.About 16kg of cannabis buds, valued at £230,000, were inside, along with £250,000 of production equipment including lamps.
DI De Curtis added: ‘This is the largest and most sophisticated cannabis factory I’ve ever experienced and I am delighted that we have been able to take such a large amount of illicit drugs off the streets of Lancashire before they are able to reach vulnerable people within our communities.’