Decriminalising drugs would have little impact on the extent of their use across Britain, the Home Office has admitted for the first time in decades.

Ministers are expected today to take the first steps to ban the sale of “legal highs”, but a Home Office report has said that tough laws have little effect on adult drug taking.

The report examined various countries’ drug policies, including those that have harsh penalties and those such as Portugal, where possession of drugs for personal use has been decriminalised. It found no evidence that drug use was affected by the adoption of either tough or soft drug policies and the severity of penalties had little effect on drug use.

The report’s controversial findings on decriminalisation undermine a long-standing