Hounslow police have increased their assault on the growing number of illegal cannabis factories despite the drug cartels becoming wise to some methods of detection.

Previous raids by the Borough's elite taskforce have uncovered houses that had converted every room of residential houses to cultivate marijuana plants, but gangs are now operating smaller operations and switching the electricity off at night to avoid detection.

Because of the busy flight path for Heathrow airport, police helicopters with thermal imaging devices are restricted to the areas and times they can fly and the clever vietnamese gangs are running electricity during the day. This is highly dangerous according to an electricity board official: "They bypass the electrics so they can generate massive amounts of energy to help grow the plants, but this is a huge hazard and has caused fires before in these types of set-ups."

The gangs, have also been linked to murders and human trafficking, and can make millions of pounds by renting houses from unsuspecting landlords and converting them into sophisticated cannabis factories.

Steve Gilbert, from Hounslow CID said: "The landlords are happy as long as they get paid each month. The people growing the drugs have obviously realised that we can use thermal imaging to find them, and this house has scaled down their operation."

The raid in the early hours of Wednesday morning discovered copious amounts of marijuana being grown on the first floor and in the roof of a three bedroom house at 20, Norman Road, Hanworth.

Downstairs looked completely normal and had incense sticks by the front door to mask the smell if the authorities or visitors came knocking.

Upstairs however was packed to the rafters with the plants, compost and cultivation equipment.

The house was rented from the same agent as many other raided houses. The name on the tenancy was a Mr Nguyen, which is as common name in Vietnam.

Marijuana plants normally take around 13 weeks to cultivate, but police believe the Vietnamese gardeners' have got this time down to just eight weeks.

Detective Inspector Jeff Minns, said: "Once again, police have reacted swiftly and positively in dismantling another segment of the apparently growing cannabis factory problem. The key point here is the information coming to police was responded to almost imediately, which empahasises even more the cooperation that exists between police and local community members.

Debbie Liddle who lives opposite the house was shocked. She said: "We had no idea what was going on there, but it was all very exciting on Wednesday morning to see the police arrive and bash down the door. I even rang my mum to let her know!"

Neil Blay lives in the house next door. He said: "It's normally two Vetnamese men that come in and out. There was one woman too and they were all in their 20's with a normal slim oriental type build.

"They've been here about a year and I've only seen them half a dozen times. They bricked up two of the windows just after they moved in which we thought was a bit odd."

Police admit flight path restrictions are a hindrance, but say they have many other ways of detecting these houses.