Police uncovered "farms" at houses in Colne, Bolton and Keighley which were responsible for producing 1,400 skunk plants with an estimated value of £1 million every 12 months in a major operation in November Bolton Crown Court heard.

Four men and two women admitted their part in the operation, pleading guilty to conspiracy to produce cannabis at an earlier hearing.

Four were arrested at a house in Bolton, with the other two arrested in Nelson and Keighley after an extensive surveillance operation by police.

Police also found extensive hydroponic cultivation equipment, £4,000 in cash and evidence of £5,000 transfers to Vietnam.

The gang's organiser, Anh Nguyen (20) was jailed for four years. The court heard he was paid £500 a month for his role, which involved visiting several properties and arranging their tenancy agreements.

He used a fake Czech passport and said he was told what to do in phone calls from a man in London, who he knew only as Mr Tung.

Anh Nguyen was also involved in a larger operation in Newcastle, which was smashed after police raided 11 addresses in the north-east and seized thousands of plants.

Judge Steven Everett told him: "You are in an entirely different league. You are much closer to the top of the tree. If you weren't one rung from the top of the ladder, you weren't very far from that rung."

He said that Anh Nguyen played a part in the subjugation of the other defendants, which was partly responsible for their miserable situation.

He added: "You played a sophisticated part in a sophisticated operation."

Tai Lam (33) was labelled as a "manager" by the judge, and jailed for 18 months, while Ly Tran (27), Tu Nguyen (35), Son Nguyen (38) and Huong Nguyen (22) were labelled as "gardeners" and were each jailed for 12 months.

Huong Nguyen claimed that she was sent to England from Germany, and when she tried to get out of the business she was raped and received death threats.

She, Ly Tran, and Tran's partner Tu Nguyen, said their involvement was restricted to cooking and cleaning for the people who grew the plants. Son Nguyen, who had only been in the country four days when he was arrested, said his duties were limited to guarding a house.

None of the four gardeners were paid for their roles, other than being given food and accommodation.

Judge Everett said they had been exploited by people who used them to make "enormous" profits.

He said: "Those at the top used the four of you as general dogsbodies, leaving you with no option but to do what you were asked."

All except Huong Nguyen were illegal immigrants, and all will be deported to Vietnam when their sentences end.

Speaking outside court, Det. Chief Insp. George Fawcett said: "Just because these particular cases have concluded today, it does not mean the end of the matter. We will continue to crack down on people involved in the cultivation of cannabis."