The regime at a prison was one of the worst of its type to be seen by a watchdog, according to a scathing inspection report.
Inmates at HMP Hindley were regularly locked in their cells for more than 24 hours, leaving them unable to attend work or education.
In a report that will re-intensify controversy over the state of prisons in England and Wales, HM Inspectorate of Prisons found levels of violence at the Category C facility near Wigan to be "very high" for the type of prison.
A survey of prisoners indicated it was "far easier" to get hold of drugs than clean clothes, bed sheets or books.
Residential wings and landings were dirty, with inspectors finding mould and fungus, while prisoners struggled to access basic entitlements such as showers.
Peter Clarke, Chief Inspector of Prisons, said: "The regime at Hindley was one of the worst, and possibly the very worst, that inspectors had ever seen in this type of prison.
"The length of time for which young adults and adults alike were locked up was, in our considered view, unnecessary, unjustifiable and counterproductive.
"Almost every aspect of prison life for the prisoners was adversely affected by the regime."
Hindley held 515 male inmates at the time of the inspection in July.
Locking inmates up for "wholly unreasonable" lengths of time had not altered the fact that nearly half of prisoners said it was "easy" to get hold of illegal drugs, the report said.
New psychoactive substances - previously known as legal highs - were a "major issue" at Hindley.
Mr Clarke said: "To make progress, there needs to be a very clear recognition of what is good at Hindley, and also where there needs to be fundamental change.
"Many examples of good practice could be found in the chaplaincy, education and health care. The same could not be said for residential areas.
"There is no good reason why Hindley should not become a safe, decent and respectful prison."
Peter Dawson, director of the Prison Reform Trust, said: "We are used to dreadful inspection reports about dilapidated, overcrowded Victorian prisons. HMP Hindley is none of these things, and this damning verdict is all the more troubling as a result."
Earlier this month, thousands of officers stopped working in protest amid soaring levels of violence and self-harm behind bars.
The Government has unveiled a wide-ranging blueprint for prison reform, including an additional 2,500 officers and measures to stop drones dropping drugs into jails and to block the illegal use of mobile phones.
Justice minister Sam Gyimah and Michael Spurr, chief executive of the National Offender Management Service, will appear at the Commons Justice Committee on Tuesday.
In response to the Hindley report, Mr Spurr said: "Since the inspection a detailed improvement plan was developed to address the weaknesses identified by inspectors and this is being closely monitored.
"Progress has been made to improve safety and purposeful activity with more prisoners engaged in high quality work and training opportunities.
"Additional staff have been transferred into the prison to support the improvements required and the governor is working closely with Greater Manchester Police to tackle gang behaviour and violence in the prison."