Report on HMP Garth comes as European watchdog says none of British jails it visited could be considered safe for inmates or staff

More than 80 prisoners in fear of their safety have sought sanctuary in a special unit in a Lancashire jail inspectors say is the most violent they have visited in recent times.

Their report into Garth prison, near Leyland, said the levels of violence at the “very unsafe” prison were linked to a major drug problem with £40,000 worth of drugs recently uncovered and 350 litres (616 pints) of “hooch” found over the Christmas period.

The document comes as a European human rights watchdog, the Council of Europe’s committee for the prevention of torture, reported that none of the prisons it visited in Britain last spring could be considered safe for prisoners or staff.

The committee also said it believed that “alarmingly high” official figures on violence inside British jails actually understate the true severity of the situation.

The deepening crisis inside prisons is accompanied by fresh evidence that the privatisation of the probation service continues to produce “troubling” consequences. The chief inspector of probation reveals that probation supervision for a quarter of low-risk offenders in Gwent now amounts to a phone call every six weeks.

It is expected that further progress on the prisons and courts bill – unveiled by the justice secretary, Elizabeth Truss, last month as the biggest overhaul of prisons for a generation – will be delayed until after the 8 June general election.

Peter Clarke, the chief inspector of prisons, says in his report on Garth prison that the Lancashire jail, which holds 800 male inmates mainly convicted of serious violent or sexual offences, had gone into a steep decline in the last three years.

Inspectors found during their inspection in February that levels of violence had increased substantially, with many incidents linked to drugs, gangs and debt. Two-thirds of prisoners told inspectors they felt unsafe.

About 85 prisoners were – in addition to sex offenders – held separately because of fears for their safety. A number of prisoners were refusing to leave their cells because of the tense, and occasionally menacing, atmosphere on the wings.

“The prison was one of the most unsafe we have been to in recent times. Violence and drugs dominated the prisoner experience. A new governor and deputy governor were appointed immediately,” said Clarke.

Michael Spurr, the head of National Offender Management Service, responsible for prisons and probation in England and Wales, said the deterioration in safety at Garth was a serious concern and reversing it was a top priority. He said the new senior management team at the prison would be supported by additional staffing and resources and an improvement plan.

But the situation at Garth is not exceptional, according to the report by the delegation from the European committee for the prevention of torture.

They said that unless determined action was taken to significantly reduce the prison population in England and Wales, the improved regimes envisaged by the government’s roverhaul agenda would remain unattainable. They expressed “deep concerns” about the amount of “severe generalised violence” evident at each of the prisons they visited and said budget cuts had reduced the number of frontline staff by 30%, compromising operational safety because of low staffing levels on wings.