Oxfordshire's top drugs officer said the county is on the verge of a mental health epidemic unless more is done to tackle cannabis abuse.

Pc Leigh Thompson, the county's drugs coordinator, last night called on the Government to reclassify the drug after statistics showed misuse had soared in Oxfordshire in the past four years.

The number of cannabis farms and people caught using the drug has risen since the Government downgraded cannabis from a Class B substance to a Class C drug in 2004.

Pc Thompson said: "I definitely want to see cannabis go back to being a Class B drug.

"If you have an underlying mental health illness, then smoking cannabis will bring it out.

"There has been an increase in the use of cannabis since reclassification because people thought it was legal."

"But smoking cannabis is still a criminal offence, and the drug causes misery. I've seen a family in Oxfordshire where a young person has been smoking cannabis and it's destroyed them."

In Oxfordshire, 341 people were cautioned for cannabis possession since last April.

During the same period in 2006, officers issued a total of 301 cautions.

Since reclassification, the number of cannabis factories discovered across the county has rocketed, from none in 2005, to 25 last year.

Marjorie Wallace, chief executive of the mental health charity SANE, said: "While many people can smoke a joint with no long-term effects, for some young people regular use can double their risk of developing a condition in which a person may hear voices and experience strange thoughts and paranoid delusions."

Glenda Daniels, of the drug charity Oxfordshire User Team, said: "It does not matter what class a drug is - it's still going to be used."