The plant most popular with illicit farmers is actually skunk, a hybrid cannabis plant specially bred to be more potent: whereas standard cannabis contains about one to five per cent of THC (tetrahydro- cannabinol - the chemical that provides the "high"), skunk can contain as much as 30 per cent THC, making it dangerous.

Although the smoking of cannabis has been downgraded by the Home Office, distribution and manufacture is still a serious crime. Detective Chief Insp Jon Chapman from the Hertfordshire Serious Organised Crime group, told The First Post: "These skunk factories tend to be in suburban premises and are designed to turn out industrial quantities of drugs. A single crop can be turned round in 11 weeks and show a return of £80,000. It's a serious crime with serious social implications.

Police raids on cannabis farms,

invariably set up in rented suburban homes, have become so common they are rarely reported. But taken together, they show a huge rise in production. In Greater London alone, police have been raiding two factories a day. Nationwide, the figure for raids is well over 1,000 a year.

Harry Shapiro at the charity Drugscope says that as soon as one cannabis farm is closed another opens. "Sixty percent of cannabis consumption in the UK is now home-grown and the percentage is rising all the time."

It is no coincidence that the front man arrested in the Bricket Wood raid (left) was Vietnamese. Gangs of illegal immigrants from Vietnam specialise in setting up and running the farms, using children to tend the crops. Which is why immigration officers always accompany the police on their raids.