POLICE netted north Wiltshire's largest ever seizure of cannabis plants after beat bobby PC Steve Dixon sniffed out a warehouse drugs factory.

PC Dixon and colleague Sergeant Nic Shorten first got scent of the 214 cannabis plants as they walked down an alleyway connecting The Pippin to Broken Cross, Calne, while out on a routine patrol.

The smell grew stronger as they walked past the houses either side of the alleyway and reached a small warehouse at the bottom of the lane where it opens out onto the Broken Cross road.

But they were unable to pinpoint the source until PC Dixon discovered an extractor fan vent at the back of the warehouse from where they could clearly smell cannabis fumes.

"As we walked around the warehouse we could smell it. It has a distinct sweet smell," said PC Dixon.

The discovery was made two weeks ago and officers kept the warehouse, which is just yards from homes, under surveillance in the hopes of catching the person leasing the building.

When there was no sign of the person returning, police obtained a search warrant from magistrates in Chippenham and raided the building on Friday.

Uniformed officers and members of Wiltshire's proactive drugs unit swooped on the warehouse, seizing the 214 cannabis plants with an estimated street value of up to £33,000.

The cannabis was being grown in a specially-built wooden shed within the warehouse. Another shed was found half-built alongside it.

When police opened the main door to let light in, the sweet herbal smell was so strong it could be smelt at the bottom of the street.

The sheds were lined with plastic sheets and high-powered lights hung from the ceiling. Timers connected to thermometers controlled room temperature.

An extractor fan in the corner was wrapped in filters, which police believe was intended to reduce the smell. The plants were potted individually and were neatly arranged on raised wood-chip pallets to improve heat circulation.

From the size of the cultivation, police believe the plants were probably grown for profit and intended for circulation on the streets of Calne.

PC Dixon said: "I was just amazed at the size of the cultivation. It's the largest amount I have ever seen grown in one place. I am really pleased about the find, which will stop this cannabis getting onto the streets."

Sgt Shorten said in his opinion it was not an amateurish operation. "This was a well thought out, industrious cultivation, which must have required detailed planning," he said. "This sends out a clear signal that whatever the Government says about possession, the law remains unchanged as regards cultivation and dealing."

Police want to speak to a man believed to have been leasing the warehouse for the last three months in his mother-in-law's name but they are not sure how long the cannabis plants have been there.

The plants, which can grow up to 8ft, were only 3ft tall and police believe they were probably only six weeks old.

When the plants are ready to harvest the top third of the plant is stripped, dried and then weighed into bags ready for sale.

Neighbours noticed the lights were on all night and police have discovered the warehouse's electricity bill was enormous, suggesting it was a round the clock operation.

One Broken Cross resident said: "We're not really surprised at all we had our suspicions.

"We often saw people go in with nothing and come out with boxes.

"We saw some men go in with boiler suits and wood strips. We thought maybe they were running a carpentry business but we never heard any noise."

Another neighbour said: "I never suspected anything. But my husband said he always saw someone coming out of the warehouse before he went to work in the morning at 7am."

Officers wearing rubber gloves removed the plants from their pots and placed them in paper sacks. They will be stored at divisional headquarters in Melksham until after the case goes to court and then incinerated.

Six plants were sent to the police laboratory in Chepstow to confirm the type of cannabis.

PC Paul Rhys, of the drugs unit, said: "We don't come across many cultivations on this scale but that doesn't mean drug farms of this type are isolated.

"I think quite a few a people out there are growing plants whether it's two or three for personal use or two or three hundred to mass produce and sell on.

"The officers on the street were doing what they are supposed to do. They smelt cannabis and thought they'd better check it out.

"It was quite a lucky find but you need a bit of luck to make things happen."

In January, police seized £15,000 of cannabis after finding a drug farm in an empty house in Charter Road, Chippenham.

Despite recent Government legislation, cannabis is still a class B drug and carries a maximum sentence of 14 years for intent to supply.