Ministers were accused today 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 per cent 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."
Shadow justice secretary Dominic Grieve said: "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.
"Unless we have abstinence-based treatment then our ability to reduce crime after release is going to be very limited because these people, on release, will simply drift back into the drug world from which they came and will start committing crimes again."
He told the BBC Radio 4 Today programme that tackling drug dependency would be "one of our priorities" if the Tories won the next general election.
But he conceded that, although changing the policy could save money in the long term, immediate budget constraints could make it difficult to act.
"We have committed ourselves to putting in the necessary resources to expand prison places to cure overcrowding. It is essential because you can't deliver the training and rehabilitation and I have to say I think that may be one of the reasons methadone treatment of this kind is being favoured in overcrowded prisons.
"Although it may be more difficult to deliver than it was three, four, five years ago, we are committed to achieving that."
A spokesman for Justice Secretary Jack Straw said: "Dominic Grieve's comments about the Prison Service are both wrong and a disgraceful slur on the thousands of public servants who work so hard in our prisons.
"Moreover, he doesn't appear to understand how the Prison Service works. Prison officers do not prescribe methadone - that is the job of medical professionals.
"It is simply untrue to suggest that methadone is 'doled out' for no good reason.
"It is also a fiction to suggest a split between the two departments - this is categorically untrue."
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