Heroin addiction is being tackled using methadone as a substitute
Government in-fighting is undermining efforts to tackle prisoners' heroin addiction, a former drugs tsar says.
Mike Trace said prisoners were being prescribed the addictive heroin substitute methadone instead of being encouraged to get drug-free.
He said the Department of Health wanted to control treatment, meaning inmates were being diverted from Ministry of Justice schemes to get them off heroin.
The government said more prisoners were "getting the treatment they need".
Almost 20,000 inmates in England went on heroin substitute methadone last year - a treatment funded by the Department of Health.
This was a rise of 57% on the previous year.
Mr Trace, formerly a drugs adviser to Tony Blair, said: "When they (inmates) see the healthcare professionals they are offered, sometimes the only choice they are offered, is a prescription of some type, which means their motivation to try to remain drug-free can be undermined.
"We see that regularly on a week-by-week basis."
Health ministers had agreed to spend £40m on drug services in prisons "not because they love methadone, it's because they want to take control of prison drug treatment", he added.
Mr Trace heads a charity running drug rehabilitation programmes in England's jails.
BBC home editor Mark Easton said: "Methadone can be an effective tool in helping heroin addicts conquer their addiction but critics argue that too often drug services use it as an easy option and are not ambitious enough in getting users 'clean'."
In a joint statement, the Department of Health and Ministry of Justice said: "It is categorically untrue to say methadone is used as any sort of control mechanism. Decisions regarding treatment are clinically based.
"The programme includes abstinence, but all treatments are aimed at getting the person off drugs.
"The rise in prisoners getting methadone treatment means more prisoners are getting the treatment they need and there has been significant investment in prison clinical drug treatment to help this happen."
Shadow home secretary Dominic Grieve said there has been a shift in policy from abstinence to dependency.
He told the BBC: "Methadone-based treatment may be suitable for someone in prison for a short time on remand and has a serious drug problem.
"But what is happening is that effectively the prison service has become content in doling out methadone as an alternative to tackling the underlying problems these people have - it's quite wrong."