The long battle to break the link between drug addiction and criminal behaviour is being won, a ground-breaking study into the long-term success rates of treatment programmes suggests.

Nearly half of all addicts who participated in drug courses in 2005 have been found to be free from addiction and no longer committing crime four years after leaving treatment. For those with cannabis or cocaine habits the success rates are as high as 69 per cent and 64 per cent respectively.

The success rate among the 41,000 drug users involved in the National Treatment Agency for Substance Misuse and Home Office study is higher than previous government figures for reoffending rates for addicts on court rehabilitation orders.

These findings, discussed at a National Treatment Agency (NTA) board meeting yesterday, will add to the pressure on ministers to pull back from cutting millions of pounds of funding for drug treatment. They will also influence the Coalition's drug strategy due to be launched at the end of the year.

Paul Hayes, chief executive of the NTA, said: "These findings are very exciting because they help us define more accurately what success looks like for drug treatment. Typically, a user coming into treatment is heavily addicted, in poor health and has low self-esteem."

In an international first, the NTA tracked the post-treatment outcome of drug users over a four-year period, discovering that almost half of those discharged in one year subsequently demonstrated sustained recovery from addiction.

Of the remainder discharged in 2005-06, about half directly returned to treatment, and a further one-third were redirected back into treatment through contact with the criminal justice system. Of those who left treatment but subsequently reoffended using drugs, 65 per cent went back into treatment.

The NTA matched four years' worth of National Drug Treatment Monitoring System (NDTMS) data with drug test records and the Drug Interventions Programme to evaluate the long-term outcomes of drug treatment for 41,475 people who left drug treatment in England in 2005-06.

Hayes said: "Experts agree that heroin and crack cocaine users take several years to overcome addiction, and need repeated attempts before they do. This means annual statistical reports of numbers in drug treatment can present a distorted picture of a system that is subject to a steady ebb and flow of people coming and going over a longer timeframe."

He added: "Now, thanks to our extensive NDTMS database, we can follow the treatment journey of individuals over successive years and demonstrate that positive change and recovery from addiction is possible."

Although, there is no international equivalent study, the NTA results compare favourably with long-running studies about the prospects of recovery. They also compare favourably with figures for the short-term outcomes of prisoners on drug rehabilitation and supervision orders, which show that two-thirds go on to reoffend.

Mike Trace, a former deputy "drug tsar" under Labour, said these success rates would be even greater if the Government could devote more funding to intensive drug-treatment programmes in prison.

Commenting on the study, Martin Barnes, the chief executive of DrugScope, said: "There is no silver bullet to overcoming addiction - drug dependency is a relapsing condition and it can take several attempts at treatment before a sustained recovery is possible. This study is helpful in building a clearer picture of what the treatment system has achieved, but we need to be even more ambitious."

A spokeswoman for the Ministry of Justice said: "The Government is committed to tackling the problem of drugs and to introducing a rehabilitation revolution that will ensure that sentencing for drug use helps offenders come off drugs, and explore alternative forms of secure, treatment-based accommodation for drug offenders."

Celebrating lives transformed

"Hi, my name's Yana and I'm an addict," says a 37-year-old woman hunched over a table on a podium in central London.

"Hi Yana," shouts the audience of counsellors, prison officers, probation workers, friends and fellow addicts.

Her obvious nervousness and vulnerability add to a heart-warming moment of redemption and epiphany in a day when scores of offenders reaffirm their new drug-free lives.

Yet only five years ago, Yana Stewart would have happily smacked you in the face as soon as she looked at you.

In a life marred by misery, it was Yana's single piece of good fortune to find a place on an intensive drug treatment programme run by the Rehabilitation for Addicted Prisoners Trust. The charity's abstinence-based approach means that three quarters of those who complete the course stay off drugs and stay out of prison for life.

Yana went on to outline her life of addiction: "My mum was a drinker. She was Scottish and liked a drop of whisky. I had my first can of Tennent's [lager] when I was 12 - I liked it and I never stopped."

Yana says the drink helped her escape her destructive family background.

By the time she was 17 Yana had two young children with a boyfriend who was hooked on drugs and in and out of prison. He died when he fell from a 10th-floor window during a burglary.

Shortly after her 18th birthday, Yana received her first prison sentence for assault and her children were taken into care.

The turning point came in 2007 when she was jailed for three years for beating up her mother during a crack cocaine-fuelled row.

"I was sitting there in my cell for beating up my own mum and it made me feel ashamed."

It took a life-changing event before Yana could finally break her addiction.

"I got a call in prison to say that my daughter had been hit by a car and she was in hospital in a coma. It was devastating."

Yana's journey was one of scores of harrowing stories recounted by addicts.

Yana has been out of prison for six months.

- INDEPENDENT