Drugs squad detectives were today stepping up the hunt for a cannabis factory boss linked to five farms in Bradford.

Nguyen Van Duc, 53, is said to be a main player in the production of cannabis in the district. He has been on the run for three years but police believe he could still be in the Bradford district.

He is wanted in connection with the setting up and running of five cannabis factories, in Great Horton, Girlington and Lidget Green.

Last week a fellow Vietnamese national, 31-year-old Sen Trinh, described as an assistant manager, was jailed for four years for his involvement in two of those farms, and others in Manchester.

The pair were among an 11-strong rogues gallery of cannabis dealers being sought by police whose faces were published in the Telegraph & Argus nearly two years ago.

Six of those have now been arrested and four of them, including Trinh, have been sentenced to a total of 11 and a half years’ imprisonment. A fifth is awaiting trial.

Police are still hunting Duc and four other Vietnamese or Chinese nationals – Chiu Wing Cheung, Gai Luc, Long Nguyen and Tien Anh Dao.

PC Dave Jackson, of Bradford District Drugs Team, said: “These people are still being hunted, even though some of the offences are four years old.

“The sentencing of Sen Trinh shows that they will be found, brought before the courts and given appropriate jail sentences.

“Duc was one of the big players in Bradford – and probably continues to be – and if we find him it will have an impact on the drugs trade.

“We urge anyone who has information about his whereabouts, or any of the others who are outstanding, or anyone who has suspicions about potential cannabis farms, to come forward and tell us.”

Duc had been arrested for a similar offence in Nottinghamshire but failed to answer his police bail granted in 2007. He was featured, with Trinh, in a BBC Crimewatch appeal last year.

Detective Inspector Neil Benstead, head of the Bradford District Drugs Team, said that since early 2006, police had uncovered 140 cannabis factories in the Bradford district. But the number being found has dramatically dropped from about 15 a month two years ago to one or two a month now.

Det Insp Benstead said: “This is because of the work of the communities in identifying them and the neighbourhood policing teams and district drugs team investigating them diligently. That will continue.”

Neighbours should watch out for a strong and unusual smell, the sound of banging, closed curtains, bright lights left on and fertiliser and compost being carried indoors. Landlords are urged to request original passports of foreign nationals renting properties.

Anyone with information about the fugitives or suspects there is a cannabis factory near to them should contact the Bradford District Drugs Team via 0845 6060606, or Crimestoppers, anonymously, on 0800 555 111.