The creation of drug-free prisons in England and Wales is too expensive and not a practical option with more than half the record 83,000 jail population misusing drugs, according to a consultants' report commissioned by justice and health ministers.
The study by PricewaterhouseCoopers released yesterday also says mandatory drug testing should be abandoned for individual prisoners as it is widely viewed by both inmates and staff as "open to manipulation", with clean urine samples often being used as a currency inside jails.
The review of the £75m a year drug treatment programme in prisons concludes that notable improvements in care have followed the doubling of investment in the last 10 years.
The consultants, however, add that treatment is fragmented between prisons and the community, there is little agreement on what it is trying to achieve, the evidence for some of the courses used is weak, and there is a lack of meaningful data to measure progress.
Extra funding for the treatment services in prisons was announced by ministers in March when an anodyne summary was released, but the full report was published yesterday after freedom of information requests. The prison population hit a record yesterday of 83,171 - up 140 in the past week.
The consultants say that despite the increase in funding, providing minimum standards of drug treatment in every prison in England and Wales is not feasible with current resources. Instead they suggest that some groups of prisoners such as young offenders and older prisoners for whom treatment is likely to have a bigger impact should be given priority access so that a minimum standard can be provided.
Controversially, the consultants say this would mean excluding men in their 20s who have been convicted of more serious crimes. But they add that an exception would have to be made for inmates serving indeterminate public protection sentences for whom completing a drug treatment course is a condition of their sentence, as there will be "legal consequences if they are not considered to have been fairly treated".
In drawing up a cost benefit analysis PCW estimate that the average male problem drug user - those with a heroin or crack addiction - "costs" society £827,000 over their lifetime in healthcare and criminal justice interventions.
The consultants also suggest that where there is no evidence a specific treatment programme works then it should be withdrawn, and add that there are a number whose effectiveness is uncertain.
The prisons minister, David Hanson, said the PWC report was a useful contribution which had led to the formation of a prison drug treatment review group to oversee the development of prison drug treatment and to streamline treatment provision in prisons.
