Gordon Brown's plans to tighten the law on cannabis by increasing the penalties for possession suffered a fresh blow yesterday as the latest official figures showed the decision to downgrade the drug had been followed by a significant fall in its use.

British Crime Survey statistics showed that the proportion of 16- to 24-year-olds using cannabis slumped from 28% a decade ago to 21% now, with its declining popularity accelerating after the decision to downgrade the drug to class C was announced in January 2004.

Mr Brown asked the government's advisory committee on the misuse of drugs to take a second look at the classification of cannabis in July after reports that stronger strains of cannabis - known as "superskunk" - were dominating the British market, with startling implications for the mental health of frequent users.

At the same time three Home Office ministers, including the home secretary, Jacqui Smith, publicly admitted using cannabis in their youth. Ms Smith will have the responsibility of deciding whether to regrade cannabis as a class B drug early next year when the committee is expected to report. The change would mean the current police policy of confiscating any of the drug found and issuing a warning on the street would be replaced by arrest and a formal caution at a police station.

But the BCS figures published yesterday showed that cannabis is beginning to lose its allure with teenagers. The proportion of frequent users in the 16-24 age group, who were using the drug more than once a month, fell from 12% to 8% in the past four years.

Since cannabis was downgraded the proportion of young people using it has fallen each year from 25.3% in 2003-4 to 20.9% now. Among those aged 16 to 59, the proportion over the same period has fallen from 10.8% to 8.2%.

Earlier this week, a Joseph Rowntree study found that heavy cannabis use exacerbated the existing social problems of vulnerable young people but those in further and higher education who lead relatively stable lives managed their cannabis use with fewer negative effects.

The decline in cannabis consumption prompted a call from the independent UK Drugs Policy Commission to take decisions about drug classification out of the hands of ministers. Dame Ruth Runciman, the commission's chief, said: "We do not believe the credibility of the current system or the clarity of message has been enhanced when, in just the space of seven years, five home secretaries have sought one way or another to address the classification of cannabis."

Good policymaking had been overshadowed by "politicking", and much of the confusion among public and politicians stemmed from misunderstandings about the classification system. The commission also says there is no evidence a drug's classification deterred use, and suggested that instead of tinkering the whole classification system should be placed outside the direct control of politicians.

The Home Office said yesterday the "widespread growing use" of police powers to issue street warnings was also a likely contributory factor to a 54% increase in cannabis seizures in 2005. The number of people dealt with each year by the police for possessing cannabis has doubled since the drug was downgraded.

Claims that "superskunk", which is up to 10 times stronger than imported resin, was dominant were also undermined by police and customs seizure figures. The latest statistics published yesterday showed that 69 tonnes of cannabis were seized in 2005 of which 49 tonnes were the traditional resin. Although herbal cannabis seizures, of which "superskunk" is one variety, were up by 76% - reflecting the growth of a homegrown cannabis industry - they still only amounted to 20 tonnes, less than one-third of the market.