The drugs charity DrugScope said that the group of six inmates and former inmates who used heroin and other opiates were on the verge of settling out of court with the Prison Service.

The case, alleging that the “cold turkey” withdrawal treatment they were forced to undergo amounted to assault, was scheduled to start at the High Court today.

The size of the payouts under discussion has not been revealed, but the compensation levels are due to be finalised tomorrow or on Wednesday, legal sources said.

Mr Justice Langstaff gave the go-ahead in the High Court in May for a full hearing of the case. It focused on six test cases chosen from a pool of 198 claimants.

When finally resolved this week, all 198 may be handed compensation by the Prison Service, with sums running potentially into tens of thousands of pounds.

Mr Justice Langstaff said in May: “The claimants complain that they entered these prisons in a state of addiction. All claim that their treatment was handled inappropriately and so they suffered injuries and had difficulties with withdrawal.”

The barrister for the claimants, Richard Hermer, told the court at the time: “Many of the prisoners were receiving methadone treatment before they entered prison and were upset at the short period of treatment using opiates they encountered in jail. Imposing the short, sharp detoxification is the issue.”

The prisoners were bringing the action based on trespass, because they say they did not consent to the treatment, and for alleged clinical negligence.

They also claimed human rights breaches under Articles 3 and 14 of the European Convention on Human Rights, which ban discrimination, torture or inhuman or degrading treatment or punishment; and Article 8, which enshrines the right to respect for private life.

A Home Office spokeswoman declined to react to the charity’s claim that there would be an out-of-court settlement.

Drugscope claims that treatment in jails has suffered cuts in the wake of the NHS budget crisis. A key project to get prisoners off drugs, the Integrated Drug Treatment System (IDTS), has had this year’s proposed budget cut by 60 per cent from £40 million to £12 million, the charity claimed.

The charity’s chief executive, Martin Barnes, said: “A major plank of the Government’s drug strategy is the reduction of drug-related crime through effective treatment for drug-misusing offenders.

“For many of the estimated 39,000 drug-using prisoners, prison can offer an opportunity to start the treatment journey.These cuts in spending jeopardise the future of effective prison drug treatment at a time when Home Office figures show reoffending rates are on the rise.”

He added: “A failure to tackle drug dependency among offenders risks repeating the cycle of drug use and crime.”

The charity said that the funding allocation meant the IDTS would go ahead in just 12 per cent of prisons in England and Wales.

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