A growing number of illegal cannabis factories hide under the busy Heathrow flightpath, where police helicopters with thermal imaging devices can only fly in restricted areas and time slots.

The thermal cameras detect the huge amount of electricity the drug cartels use to create make-shift greenhouses for the marijuana plants.

But the criminals have become wiser, and are now switching the electricity off at night to avoid detection.

Where Hounslow police's elite task force previously uncovered houses with every room full of marijuana plants, gangs are now running smaller operations.

An electricity board official said: "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."

A 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 the house was 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 cannabis plants, compost and cultivation equipment.

The house was rented by a Mr Nguyen from the same agent as many other raided houses.

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."

Marijuana plants normally take about 13 weeks to cultivate, but police believe the growers have got this time down to just eight weeks.

Police admit flight path restrictions are a hindrance, but said they had plenty of methods to detect these houses.