CONVICT drug addicts at a jail near Plymouth are being given a heroin substitute through vending machines.

Dartmoor is among a number of prisons to have installed machines automatically dispensing methadone.

The machines, which allow prisoners to access the drug directly by scanning their fingerprint or iris, are operating in 57 prisons.

A £4 million scheme will see the equipment machines installed in half the 140 prisons in England and Wales.

Shadow justice secretary Dominic Grieve, who uncovered details of the scheme, said it amounted to an "admission of failure" in attempts to get addicts clean.

The total cost of the machines exceeds by £1 million the amount spent on an abstinence programme aimed at getting addicts off drugs, he said.

He said: "The public will be shocked that ministers are spending more on methadone vending machines than the entire budget for abstinence-based treatments.

"Getting prisoners clean of drugs is one of the keys to getting them to go straight.

"We need to get prisoners off all drug addiction – not substitute one dependency for another. The Government's approach of trying to 'manage' addiction is an admission of failure."

It comes amid wider concerns about the 'escalating' use of the heroin substitute to treat drug addicts generally.

There is also the cost, which is thought to be between £3,000 and £4,000 per patient, per year.

Methadone prescription is official policy for tackling heroin withdrawal. Its supporters say that despite its disadvantages, it gives the best hope of breaking the chaotic cycle of hardcore heroin use.

It has been compared to a nicotine patch for heroin addicts, providing a slow, steady delivery that allows addicts to stabilise their cravings.

Usually taken as a syrup once a day, methadone helps replace frequent daily injections of heroin, and the accompanying crime that addicts commit to pay for it.

But although methadone has been shown to reduce chronic heroin use and the crime that accompanies it, the drug is less successful at stopping people from taking drugs altogether. Some continue to use heroin, and use methadone as a "top up" drug.

Critics say methadone just replaces one dependency with another, and some others say it can be even harder to quit than heroin.

It is also highly toxic. It is possible to overdose on methadone, leading to a coma or even to stopping breathing.

A Department of Health spokesman said: "The department spends around £240million on offender health. £40million is for the integrated drug treatment programme in place in prisons, and methadone dispensers are only a small part of this.

"Clinical professionals decide what treatments are best for individuals, but ultimately, all of them are aimed at getting people off drugs.

"Methadone dispensers are a safe and secure method for providing a prescribed treatment. They can only be accessed by the person who has been clinically assessed as needing methadone and that person is recognised by a biometric marker, such as their iris.

"This means that the treatment can only be given to the right person, at the right time, in the right dose. A health professional then oversees the person taking the treatment.

"The National Offender Management Service is also taking robust action to reduce the supply of and demand for illicit drugs in prisons. The level of drug misuse in prisons – as measured by the random mandatory drug testing programme – has declined by 63 per cent since 1996/97."