One of the Metropolitan police's most senior officers is backing a major report which will argue that the government needs to rethink its drugs policy, and possibly take the issue out of the authority of the Home Office. The study, which will be released next month, will say that Britain's drugs problem should be treated predominantly as a medical, rather than a criminal, issue.

The approach has won the backing of the force's assistant commissioner, John Yates. He is one of a number of senior professionals, including doctors, on a commission chaired by the politics professor Anthony King and launched by the Royal Society of Arts.

The report - coming after the Conservative party leader, David Cameron, has faced claims that he is not fit to be prime minister due to his use of cannabis at the age of 15 - will urge a rethink in the way in which drug treatment is offered. It will argue that it is illogical, expensive and inefficient to offer treatment primarily to those found to be using drugs when they have committed a crime.

Many of those forced on to drug treatment courses are not yet ready to go on to them, either due to psychological or addiction issues, the report will say. By contrast there are many drug addicts that have not committed crimes, or not been caught committing crimes, that find it impossible to get on to drug treatment courses, due to their lack of availability.

One source said there was a failure of logic to suggest that drug treatment should be made available to those that have committed crimes.

There is strong evidence that many of those pushed on to drug referral programmes fail to complete the course.

The report will also call for a reclassification of drugs, so that the classification reflects the damage to the user's health, rather than the potency of the drug.