The UK drug policy commission's case for decriminalisation rests on the evidence of the limited deterrent effect of Britain's 40-year-old drug law, with an estimated 3 million people having used an illicit drug in the past year.

Its report says that since 1971's Misuse of Drugs Act came in the nature of drug use in Britain has changed radically and the legislation was never designed to deal with such high levels of use nor the rapid development of new drugs.

Under this legislation more than 160,000 people a year are being given cannabis warnings and a further 42,000 in England and Wales sentenced for possessing drugs. Enforcement focuses particularly on the young and some ethnic groups in particular areas, with more than half of all police stop-and-search activity aimed at finding small amounts of drugs.

The scientists and drug experts say the risk of being caught possessing drugs is very low compared with the prevalence of drug use overall and the point has now been reached where the drug laws are largely seen as an irrelevance by those who break them.

"Opponents of depenalisation and decriminalisation raise concerns about the message that a change in the law would send the public, particularly young people. We recognise that the law expresses the sort of society we wish to live in. But the law relating to the possession of drugs has become discredited to such an extent that any usefulness in setting a moral position has in many situations become largely ineffectual," the report says.

It argues that the "roof has not fallen in" across parts of Australia, the US, Portugal, the Czech Republic and some South American countries which have gradually moved towards a more decriminalised approach to personal possession. "Prevalence and consumption have not increased in these countries to any significant extent. Some experts indeed argue that these reforms have led to decreasing problems."

The fresh approach advocated by the commission says the first step should be to scrap existing criminal sanctions for possession of small amounts of cannabis.

It suggests imposing simple civil penalties, such as a fine, a referral to a drug awareness session similar to the speed awareness courses motorists now face, or to a drug treatment programme if needed.

It says that the move should start with cannabis because of its relatively low level of harm, its wide usage and international developments.

"This is something which has been gathering momentum in other countries. If evaluations indicated that there were no substantial negative consequences, similar incremental measures could be considered, with caution and careful further evaluation, for other drugs," the report says.

"These changes could potentially result in less demand on police and criminal justice time and resources. Given the experience of other countries, our assessment is that we do not believe this would materially alter prevalence levels, while allowing resources to be spent on more cost-effective measures to reduce the harms associated with drug use. We would expect the net effect to be positive."

The report recommends that those who grow a small number of cannabis plants should face no or only minimal sanctions, but the experts decisively reject any wider moves to legalisation.

They say: "We do not believe there is sufficient evidence to support the case for removing criminal penalties for the major production or supply offences for most drugs. One of the lessons from the tobacco and alcohol markets is that commercialisation can lead to some disastrous consequences for the wellbeing of the public." The report warns that any move to a carefully regulated commercial market for all drugs could lead to "some hugely negative unintended consequences, and should be treated with caution".

The commission also advocates major changes in the legislative approach to drugs policy. The experts say an independent body, which could be accountable to parliament, needs to be given the power to take decisions about controlling and classifying the harms and risks involved in new drugs.

There also needs to be a new independent body to research evidence and analyse the effectiveness of drugs policy. These two bodies could replace the home secretary's current advisory council on the misuse of drugs.

The report argues that these changes are unlikely without forging a new cross-party consensus, which could be facilitated by moving responsibility for drugs policy from the Home Office to the Department of Health.

But the experts conclude that the most valuable legacy would be a change in British drug policy's relationship with evidence.

"A commitment to the use of evidence to inform which policies are adopted, combined with rigorous trials of new and existing policies, and a willingness to act on the results of this research, would go a long way towards ensuring that the UK has an effective and good value response to the use of mind-altering drugs," the 173-page report concludes.