Hundreds if not thousands of cannabis farms have been established in upmarket houses in the UK by criminal gangs who could be serving an international market.
By Winifred Robinson and Sue Mitchell
BBC Radio 4
Every weekday morning somewhere in Britain a team of police officers will be assembling to raid a cannabis farm.
The first few cannabis farms - sited in ordinary houses, stripped out and completely turned over to plants - were discovered in London six years ago.
Now they have been found in 41 of the 43 police force areas in England and Wales and in five of the eight Scottish ones.
The Metropolitan Police have raided 1,500 cannabis farms in the past two years, Strathclyde police 61 in the past nine months, and in Derbyshire police closed 200 last year.
One, on the outskirts of Ripley in Derbyshire, was only discovered when it caught fire. It was in a detached house, built in the 1930s among well-kept properties with landscaped gardens.
"It was such a shock," one old woman told me.
A fire revealed a cannabis farm at this house in Derbyshire
"The new owner had knocked on our door to introduce himself, and he seemed such a respectable oriental gentleman.
"We had no idea about any cannabis and we think it is a disgrace. It has lowered the tone of this neighbourhood. It is dragging us all down."
Gangs are targeting neighbourhoods with high quality housing, which they turn over to cannabis production.
"They avoid the social exclusion areas which attract a lot of police attention, and go for a quiet neighbourhood where people don't necessarily know a great deal about each other," says Strathclyde's Detective Chief Superintendent Stephen Whitlock.
"These criminals can afford to set up in places where homes typically fetch between half a million and three quarters of a million pounds. It is such a cash-rich crop."
To add to the challenges facing police, they often find that door handles have been rigged to live electricity cables as a booby-trap deterrent to keep out rival gangs who might steal the crop.
What the cultivations have in common is how well organised and planned they appear. Bill Stupples, from the Merseyside Drug Support Team, described an almost factory-like environment where all of the plants are at the same stage.
"In the old days you might get a local entrepreneur growing cannabis. He'd have one big old gnarled looking plant and he'd be using that to take cuttings, then there'd be a few straggly plants at different stages and finally he'd be drying the flower heads and the leaves.
Now we only ever find the middle bit of the operation - no cuttings, no drying room. But they must be out there somewhere - a huge nursery operation and a big drying plant."
This way of growing cannabis was invented by the Vietnamese and they now control an estimated 60% of the UK's cannabis supply.
Cannabis plants are intensely cultivated to generate huge profits
According to the United Nations, the opportunity to serve a world market in the drug arose when a successful government eradication programme in Morocco cut exports from that country by a third.
"They spotted a gap in the market and the network of Vietnamese criminals moved in. They found that they had an advantage as they were off the radar, initially, of law enforcement," said Commander Allan Gibson from the Association of Chief Police Officers.
One possibility is that the Vietnamese have come from Canada, which has had some success in countering similar criminal activity.
Police in the city of Surrey in British Colombia have found that the cannabis growers have an Achilles heel - their dependence on high levels of mains electricity.
A thousand suspect homes were identified, either from their very high usage or their very low usage, if they were bypassing the meter. Of 600 homes inspected so far, nine out of 10 turned out to be house cannabis farms.
The farms feature improvised wiring, heat lamps and water and regularly catch fire so the authorities have been able to use fire safety laws to force home owners to carry out costly safety work.
That has allowed them to move far more quickly and effectively than using the criminal law, and the number of cannabis farms in Surrey, British Colombia is finally coming down.
"If you can hit the house owners hard in the pocket every time, they are much more vigilant about who is renting from them," says Darryl Plecas, Professor of Criminology and Criminal Justice at the University College of the Fraser Valley.
"You may not be able to stop this but you can move the problem elsewhere. We say jokingly, let's move it on to the UK, where they are doing nothing, like we were five years ago. Let the cannabis growers go there."
The rise in the UK production comes at a time when consumption of cannabis is actually slightly down.
"There is a market for it in the UK but when you think about the amount that is being grown there is too much, it must be going somewhere else," says Detective Inspector Bill Stupples from Merseyside Police.
"It wouldn't surprise me if it was being sent to France, Spain, Germany, the Netherlands or wherever."
Listen to The Cannabis Trade on BBC Radio 4 at 2000 GMT on Thursdays 20 and 27 December.