Staff at an overcrowded, chaotic and violent prison did not always know where inmates were, an inspection has revealed.
Watchdogs warned "control and accountability" of those held at HMP Leicester was "very poor" as they issued a highly critical assessment of the Victorian jail.
The main wing was "often chaotic" and there was a "very high" rate of assaults on staff, a report by HM Inspectorate of Prisons said.
Campaigners said the findings lay bare the "catastrophic" impact of prison overcrowding, while the Government said the report was an example of the need for reforms.
Inspectors described basic procedural security as "poor", adding: "Staff often could not account for prisoners and we routinely found prisoners where they were not supposed to be."
Unlocking and movement of prisoners to activities was "poorly controlled and at times chaotic" and during spot checks inspectors were unable to obtain an accurate account of where inmates were at any one time "because they were not monitored properly", according to the report.
HMP Leicester held 325 male prisoners at the time of the inspection held in September and October last year - 50% more than the number it was built for.
The report concluded that the facility had deteriorated since it was last examined in November 2013.
There were 75 assaults in the previous six months - almost three times that found in other local prisons.
The rate of attacks on staff was almost five times that of similar local institutions, while levels of self-harm had increased.
The report also said:
:: New psychoactive substances - or so-called "legal highs" - were "too readily available", while illicitly brewed alcohol was also an issue.
:: Use of force was more than double that of comparable prisons.
:: Conditions in the segregation unit were "terrible" and it should be closed, with cells described as cold, damp, decrepit and "not fit for habitation".
:: Prisoners struggled to access basic necessities such as toiletries, clean clothes and bedding.
:: Only one probation officer managed all inmates serving indeterminate sentences and other high-risk prisoners.
Deputy chief inspector of prisons Martin Lomas said: "This is a poor report.
"We found pockets, such as the gym, substance misuse services and the work of the Community Rehabilitation Company, where the prison was operating more effectively, but much of what we inspected had deteriorated.
"Managers were aware of the problems and data was being collected, but it wasn't being used and problems were not being analysed."
He said there were "few meaningful plans to effect progress", adding: "Managers should start by making the prison safer and gaining control of basic operational routines."
Michael Spurr, chief executive of the National Offender Management Service, said: "Leicester's performance has deteriorated unacceptably and this cannot be allowed to continue."
He said a new governor has been appointed.
The "inspector reports positively on the relationship between staff and prisoners", Mr Spurr added.
Andrew Neilson, of the Howard League for Penal Reform, said: "This report sets out in stark detail the catastrophic impact of overcrowding in prisons."
Juliet Lyon, director of the Prison Reform Trust, said the findings "show a prison that has lost its way".
Last week David Cameron set out wide-ranging prison reform proposals.
The Government previously announced plans to close old Victorian jails and build nine new prisons. Only HMP Holloway in North London has been confirmed as facing closure so far.
A Ministry of Justice spokesman said the report "is an example of how our ageing prisons need reform", adding: "We take our duty of care to staff and prisoners extremely seriously and tackling violence and drugs must be a priority.
"We will be investing £1.3 billion to transform the prison estate over the next five years, to better support rehabilitation and tackle bullying, violence and drugs."