Three men have admitted their part in running a large-scale cannabis factory from an underground nuclear bunker. Martin Fillery, 45, Plamen Nguyen, 27, and Ross Winter, 30, admitted conspiracy to produce class B drugs and abstracting electricity.

Wiltshire police discovered more than 4,000 cannabis plants, capable of producing £2m worth of drugs a year, during a midnight raid on RGHQ Chilmark. The underground nuclear bunker was constructed in the 1980s to house local government in the event of a nuclear attack during the height of the cold war.

RGHQ Chilmark is no longer owned by the Ministry of Defence but is still intact, with the nuclear blast doors making the site almost impenetrable.

Police intercepted Fillery, Nguyen and Winter as they left the site, having gathered intelligence in relation to it. Officers used keys found in the men’s possession to gain entry to the bunker, which was being powered by £250,000 worth of illegally abstracted electricity.

Fillery, of Ashcott, Bridgwater, Nguyen, of Horfield, Bristol, and Winter, of Maytree Avenue, Bristol, admitted the charges at Salisbury crown court.

DI Simon Pope, of Wiltshire police, described the find as “the biggest cannabis factory we’ve ever had in Wiltshire and the south-west region”. “The isolated and secure nature of the location made the warrant particularly challenging,” he added.

“The bunker itself had approximately 20 rooms inside over two floors, and almost every single room had been converted for the wholesale production of cannabis.

“In addition to the growing rooms, there were drying rooms, nurseries and living quarters for the growers. It was a sophisticated setup with an illegal connection to the mains electrical supply. The lighting equipment alone seized from the site cost in the region of £140,000, and the setup was capable of producing a crop every six weeks.

“In all, it took approximately 10 days to completely search and clear the site.”

He said the find had stopped an “enormous amount of illegal drugs” from entering the streets of Wiltshire and the south-west.

Fillery also admitted possessing criminal property, having raised a “significant amount of assets” from his crimes, Wiltshire police said.

The three men will be sentenced on 11 August.

They were previously charged with conspiracy to hold persons in slavery or servitude, but these charges were dropped due to a lack of evidence.