Thousands of drug-addicted prisoners are having their habit "maintained" rather than halted while serving their sentence, a think-tank has said.
The centre-right group Policy Exchange said most of the 45,135 prisoners in treatment in 2008-9 were given drug substitutes for three months or more.
Its report suggested the approach by the previous government had "failed".
Author Max Chambers said it had become the "easy option for prisoners' habits to simply be maintained by the state".
He added that "little effort" was being made to "properly address [prisoners'] addictions".
The research report, called Coming Clean, suggested the setting up of the Integrated Drug Treatment System (IDTS) in 2007 had led to an increase in the frequency and length of prescriptions for methadone and a decline in shorter detoxification programmes.
But the Department of Health said IDTS was being continued.
The system is being brought in across prisons in England and Wales to provide "evidence-based treatment tailored to the needs of the offender".
"It is currently the subject of a rigorous and extensive four-year prison research programme," said a spokesperson.
They added: "Clinical guidance recommends that prisoners jailed for more than six months should not be maintained on methadone unless there are exceptional circumstances.
"All treatment, whether in or outside prison, should be aimed at getting people off drugs and maintenance can be part of that programme."
The Policy Exchange report calls for longer-term prisoners to be expected to become drug-free as part of their parole and for more focus on abstinence-based treatments.
Its report begins by saying: "It is an open secret that our prisons, traditionally thought of as secure institutions, are awash with drugs.
"The easy availability of drugs in prisons undermines treatment programmes, allows prisoners to maintain anti-social habits during their sentence, and leaves them unprepared for release and primed to reoffend."
It said there was "no doubt that significant additional funding was provided... by the previous government, attempting to both reduce the supply of drugs and to reduce demand for them through engaging prisoners in treatment programmes".
But it concluded there were still problems with the way the issue was being approached.
Director of the Prison Reform Trust Juliet Lyon said about three-quarters of people entering prison tested positive for class A drugs.
"To cut crime, save money and improve public health, more effort needs to be put into treating addicts in the community and reviewing current drug policies."