Ministers have been accused of undermining efforts to tackle prisoners' heroin addiction.
Mike Trace, a former drugs adviser to Tony Blair, said provision of the heroin substitute methadone had rocketed in an official bid to "control" drug treatment.
It meant thousands of inmates had been diverted away from other schemes to get them off heroin, he said.
Almost 20,000 prisoners in England were put on methadone last year - up 57% on the previous year - and paid for by the Department of Health.
Mr Trace - who now runs a drug rehabilitation charity - told the BBC: "When they 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."
He added that health ministers had agreed to spend £40 million on drug services in prisons "not because they love methadone, it's because they want to take control of prison drug treatment".
The Department of Health and Ministry of Justice said in a joint statement that any allegation of using methadone to control prisoners' treatment was "categorically untrue".
"Decisions regarding treatment are clinically based," the statement said.
"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."
