A more potent "skunk" form of cannabis now accounts for 70-80% of the British market for the drug, but many users are cutting down and only smoking enough to get high, the initial results of a Home Office study show.

A special meeting of the government's expert committee on drugs which is looking again at the legal status of cannabis was told by Dr Les King that the rise in the use of "homegrown skunk" - which accounted for 15% of the market in 2002 against 70% now - had been driven by the growth of "cannabis factories" run by organised crime gangs, who were often Vietnamese.

He said British-grown skunk had almost squeezed traditional imported herbal cannabis out of the market with cannabis resin mostly from Morocco still holding about 20% of British sales.

King, a technical adviser to the Home Office scientific development branch, said the skunk has an average THC content - the active ingredient in a joint - of about 12% to 14%, two and a half times that of traditional cannabis resin. He compared it to the difference in strength between beer and wine and said the amount smoked was as important as strength.

He was supported in his claim that users were moderating their intake of the more powerful cannabis by Dr Mike White from the Forensic Science Service who said it was rare for a smoker to get through an entire joint in one go. He also suggested that the potency of British skunk had actually fallen by 10% in the past two years as growers substituted quantity for quality in the face of an expanding market.

King said media reports were wrong to claim that a new "superskunk" form of cannabis 10 to 20 times stronger than traditional types was now sweeping the British market.

The two-day meeting of the Advisory Council on the Misuse of Drugs opened with its chairman, Sir Michael Rawlins, saying he had accepted a written assurance from the home secretary, Jacqui Smith, that the government still had an open mind over regrading cannabis from class C back to class B, which would again make possession punishable with a jail sentence.

Ipsos Mori polling evidence published last night exploded the myth that the downgrading of the drug's status by David Blunkett in 2004 had led to confusion about its status, with over 80% of those polled correctly saying the drug was still illegal. British Crime Survey data shows that cannabis consumption has fallen since the change.

The two-day meeting also heard evidence from the government's mental health tsar, Professor Louis Appleby, that a link between cannabis and mental illness was not yet proven. But he backed reclassification, saying there was now sufficient evidence that cannabis was a harmful drug that could contribute to a pattern of relapse and risk in mental health patients.

Appleby said he felt that many health professionals had been guilty of complacency on the issue and that reclassification would reinforce the public health message. However he had reservations about further criminalising mental health patients for using cannabis.

The scientific experts also heard calls for regrading from the Association of Chief Police Officers, which argued that the emergence of British cannabis farms and confusion over its legal status on the streets justified tightening the law.

The Home Office study now under way is the first time that there has been a concerted attempt to find out what strength and type of cannabis is actually available on the street. Police forces across the country have been asked to send up to 1,000 samples of cannabis confiscated in stop and search operations to the Home Office for analysis. King, who is the technical adviser to the six-week study, said the initial findings were based on "several hundred" samples.

The Home Office-funded study is to report to the advisory council and the home secretary in March before the government decides whether or not to regrade cannabis in April.