Criminal gangs behind a series of cannabis factories in rented homes have been chased out of the city, police said last night.

Drugs officers have hailed a clampdown against the Vietnamese gangs after a large fall in the number of cannabis farms uncovered in Oxford last year.

In 2007, they raided 24 cannabis factories and drugs worth sbout £750,000 were seized and destroyed.

But, during the past 12 months, only six have been discovered.

Police believe the criminal gangs have been forced to move to other areas of the country.

The criminals rent homes, change the locks and cause hundreds of pounds worth of damage by installing insulation, hydroponic lights, complicated water irrigation systems and rewire the electricity.

A ‘gardener’ lives in the house and operates a complicated irrigation system growing between 500 and 1,000 plants.

Five people have been jailed for working at the cannabis factories.

Although other cannabis raids have taken place since 2007, police claim only six cannabis farms found in 2008 were linked to the international drugs rings.

Pc Leigh Thompson, drugs co-ordinator for Oxfordshire, said: “They are never going to disappear because it is a lucrative business.

“But the more we as residents keep an eye out, and more intelligence is passed to the police, the more it will stop.”

In the past two years, police have worked with landlords to spot for rogue tenants.

Ajaz Raymond, manager of Beehive Lettings Ltd, in Oxford, discovered drugs gangs had converted a client’s property in Cowley Road into a cannabis factory in October.

Mr Raymond became suspicious when the tenant, from South East Asia, failed to return to his office to sign extra forms.

When he paid a visit to the house, he found the curtains closed and the locks had been changed.

He said: “Inside we discovered fertiliser and hydroponic lights.

“There were plant seeds in boxes and lots of pots and fertiliser everywhere. We were lucky we weren’t injured, because the police said often these places are booby-trapped.

“It seems we caught them really early. We have to be very careful with these guys.

“We were very vigilant and thorough and if a letting agency does its job properly there shouldn’t be any problems.

“But these guys could ruin a private landlord and their mortgage.”

Elsewhere in Oxfordshire only one cannabis farm, in Kidlington, linked to the gangs was discovered in 2007 compared with two in Botley and Banbury in 2008.

Anyone who suspects a property is being used as a drugs factory should call police on 08458 505505 or Crimestoppers on 0800 555111.