He said he remained deeply concerned over the risks of psychosis from new high-strength strains of cannabis but accepted there were arguments legalisation could pave the way for regulations to limit the strengths, generate tax income and reduce the numbers criminalised by taking it.“We need to look at it in more detail to get more evidence. One of the arguments for legalising cannabis has been that you will get purer forms of it and you can tax it so governments benefit,” said Dr James, who as registrar is head of policy at the College.“As a forensic psychiatrist, the strongest argument is decriminalising behaviour that is widespread and avoiding people getting caught up in the criminal justice system and ending up on a conveyor belt. If you can decriminalise it as an activity, you prevent that and the stigma associated with it.”Although the college believed drug policy worldwide needed to be reviewed on the basis it “had not worked”, he said there was evidence that even in states where it was legal, high-strength cannabis still “proliferated.” Dr Adrian James, the Royal College of Psychiatrists’ registrar who will chair the panel, said they would start with an “open mind” and review the medical evidence and the research from US states, Uruguay, Canada and Portugal where it has been decriminalised. The Royal College of Psychiatrists is to review its opposition to the legalisation of cannabis despite its concerns over the risks the drug poses to users’ mental health.It is setting up a panel to consider decriminalisation in the wake of more countries legalising the drug and the government’s decision this summer to make medical cannabis available on prescription. The move is critical as much of the research the government has used to keep cannabis as an illegal drug has been conducted by leading researchers from the College.Its members also hold key positions advising ministers on mental health including the chairman of the advisory council on the misuse of drugs Dr Owen Bowden-Jones, an expert on addiction, although he is not on the panel.The pressure for a rethink of laws on cannabis has grown since this summer’s controversy over the confiscation of cannabis oil that was being used to treat the epilepsy of Billy Caldwell, 12.Ex-Tory leader William Hague joined some police leaders in backing decriminalisation, saying the current law on cannabis was “inappropriate, ineffective and utterly out of date.”The Royal College of Physicians, British Medical Journal, British Medical Association and Royal Society for Public Health have also called for reform to the laws up to and including legalisation. “Our official view is that we are concerned about the health risks and we are against legalisation of cannabis on that basis but there may be arguments that outweigh the psychiatric arguments,” said Dr James. “We will look at the arguments around legalising street cannabis and the messaging we would want to put out around that.” The panel will also review medicinal cannabis, the use of which it supports when based on evidence of benefit.“We are always after new treatments for mental health problems,” said Dr James.Cannabis is the most commonly used drug in the UK, with 6.6 per cent of adults, equivalent to 2.2m having taken it even though possession carries a penalty of up to five years in prison. Suppliers face up to 14 years in jail. Dr Trevor Turner, a former vice-president of the Royal College of Psychiatrists, said it was “insane” to continue treating cannabis as an illegal drug when there was no evidence to show a causal link to schizophrenia, only an association, and when other ‘drugs’ such as alcohol and tobacco were legal.“It causes criminalisation and doesn’t make it possible to regulate for health and safe cannabis forms if it was to be legalised as is now being proved in Colorado, Oregon, Portugal and Canada,” he said. Despite agreeing to the prescription of medicinal cannabis, Theresa May and the government remains opposed to legalisation, arguing that there is strong scientific and medical evidence that cannabis is a harmful drug which can be detrimental to people’s mental and physical health. Want the best of The Telegraph direct to your email and WhatsApp? Sign up to our free twice-daily Front Page newsletter and new audio briefings.