VIETNAMESE drug growers have moved on from Oxfordshire after police seized about £750,000 worth of cannabis this year, officers believe.

Police have raided 26 properties this year where gangs have set up sophisticated drug factories, seizing more than 5,000 cannabis plants and arresting 15 people.

The majority of the raids took place in the first seven months of the year, after a surge in the number of reports of suspicious houses.

PC Leigh Thompson, drugs coordinator for Oxfordshire, believes the high intensity of the raids and their exposure has convinced the drug gangs to leave the city.

PC Thompson said: "Earlier in the year we were doing a raid every couple of weeks.

"Of course it got a little bit hot for them, and then they moved on to another area - to Milton Keynes, which has got a problem now. We have got on top of it but we are not going to be complacent."

PC Thompson said the vast majority of cannabis factories raided were producing skunk, a particularly potent type of the drug.

The gangs target rental properties, converting almost all the rooms into growing rooms and putting in a 'minder' to monitor the operation. Most of the rented houses had the electrical systems tampered with so that power could be stolen from the general supply to provide extra power for hydroponic lights, allowing the plants to flower up to four times a year.

He said: "It is a well-organised operation. If you go into a cannabis factory, every part of that house is taken over for the production of cannabis.

"In one raid the only place that was plant-free was the loo."

Five Vietnamese men have this year been jailed for a total of 91 months for their role in creating drug factories in Oxford.

PC Thompson said: "From the evidence we have got it's solely a Vietnamese enterprise."

Despite the hope that the gangs had moved elsewhere, he said there could be as many as 15 more factories in the county which the police did not know about, and urged residents to be vigilant.

Landlady Nima Mason, 38, was left £8,000 down after a cannabis factory was created in her home in Harcourt Terrace, Headington, in May by three South East Asian men and a woman who claimed to be students.

Officers found 250 cannabis plants with a street value of £28,000 at the house, and Ms Mason lost four months in rent while waiting for her electricity to be reconnected.

She said: "These people are really disgraceful. They are using people's houses and using drugs to harm people.

"I was very shocked - I could not believe it when it happened to me. The amount of drugs seized does not surprise me."

WHAT TO LOOK OUT FOR

NEIGHBOURS can often spot tell-tale signs that something is amiss, and tip off the authorities.

These include windows permanently covered, visits to properties at unusual times of the day or night, a pungent smell and vents placed in the property to remove condensation.

Acting Assistant Chief Constable George Wilson said: "In the majority of cases an ordinary terrace or semi-detached house is specifically bought or rented for the sole purpose of growing cannabis.

"The house is then kitted out with special hydroponic equipment which is then used to produce up to £70,000-worth of cannabis every 11 or 12 weeks.

"These factories can cause thousands of pounds worth of damage to the properties. They also pose a danger, as in many cases the electricity is bypassed, which could cause electrocution or even a fire."

Call Crimestoppers anonymously on 0800 555111 with information.

FACTORY FACTFILE

THE first cannabis factory in Oxford was found in early 2006, but they had already proved a blight in other cities.

In London alone, 700 factories producing the Class-C drug were shut down in 2005.

In September 2006, 17 police forces across England and Wales declared war on skunk factories, launching a series of raids.

In June this year, a hi-tech helicopter was being regularly deployed to identify factories in Ulster.

Police estimate that in the mid-1990s only ten per cent of cannabis in the UK was skunk, but in the last ten years the figure has risen to about 60 per cent.

In 2007, 119 cannabis factories have been discovered across the Thames Valley, compared with 42 in 2006.