Detectives have revealed that 375 kilos of a mixing agent for heroin were discovered at an industrial unit in Wigan.
Undercover investigators found the haul, at an undisclosed location in Marus Bridge, as part of a seven-month long hard drugs probe, spanning Greater Manchester and Merseyside, codenamed Operation Trinket.
Another 115 kilos of the same substance, used to “cut” heroin before it sold on the streets, is also thought to have been discovered at another small factory in the Birkenhead area last Friday.
Eleven warrants were executed as part of a series of dawn raids, in Wigan and on Merseyside, by police, some involving riot vehicles.
A spokesman on behalf of Operation Challenger, the regional organised crime initiative, said: “There were 12 arrests in total and six vehicles were seized.”
Detectives also impounded two kilos of heroin, which would have a street value of between £45,000 and £50,000, half a kilo of cocaine and a kilo-and-a-half of cannabis.
Designer clothing and watches, estimated to be worth £200,000, six vehicles and a further £4,000 in cash, were also seized during the raids.
Four men, two of them from Wigan and two from Merseyside, have been charged with being concerned in the supply of heroin. One of these is also accused of conspiracy to supply class A drugs. Another eight suspects have been released under police investigation while further enquiries are conducted.