Thousands of pounds worth of damage can be caused to homes

Thames Valley Police raided 123 cannabis factories during 2007, compared with 42 during 2006.

Most were in houses adapted to grow the plants.

During 2007, 43 cannabis houses were found in east Berkshire, 26 in west Berkshire, 12 in Buckinghamshire, 16 in Milton Keynes and 26 in Oxfordshire.

Acting Assistant Chief Constable George Wilson, of Thames Valley Police, said there had been a "marked increase in Vietnamese-linked cannabis houses".

Booby traps

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

"It is also known that in some cases, booby traps have been left in the houses.

"It is in the landlords' interests to keep a close eye on their property and report anything suspicious to the police."

The houses are normally adapted for hydroponics equipment and artificial lights used to cultivate up to �70,000-worth of cannabis every 11 or 12 weeks.

Indoor cannabis farms are often identified by permanently covered windows, vents protruding through the roof or a rear window, and noises from equipment and pungent smells coming from the building.