A privately-run prison has been heavily criticised for locking up inmates in cells without electricity or running water for more than two days.
HMP Doncaster, which is run by security giant Serco, is the latest jail to be slammed by Her Majesty's Inspectorate of Prisons (HMIP) after inspectors found its "performance was in decline".
The report comes as Labour hosts a summit in Westminster today on what the party calls a "growing crisis in Britain's jails".
And it coincides with a troubling report from the Prison and Probation Ombudsman into self-inflicted deaths among young adult inmates, which found s uicide risk assessments and monitoring arrangements were poor in too many cases.
Shadow justice secretary Sadiq Khan, who is hosting Labour's Prisons Crisis Summit, will tell the gathering of prison governors, officers and charities: "The Government pretends all is well in our jails. But there is a yawning leadership gap under David Cameron and Chris Grayling.
"The Tories are in denial about the scale of the crisis and offer no solutions to tackle the mounting chaos. We can't go on like this. Five more years of the Tories risks five more years of failure."
The event comes after a wave of bleak figures published by the Ministry of Justice last month revealed a leap in the number of on-the-run inmates in the last year, as well as an increase in deaths in custody and a rise in the number of jails considered to be ''of concern''.
HMP Doncaster was "experiencing real drift", according to inspectors, as levels of violence in the prison were found to be up to four times higher than typically seen in similar jails.
Some "extremely violent" incidents had been referred to the police and there had been a recent incident where a wing had been damaged by fire and vandalism.
The report also revealed some prisoners had been locked in cells with no running water or electricity for more than two days and had spent only short periods out of the cells.
Chief inspector of prisons Nick Hardwick said: " Despite some positive features, Doncaster was a prison with much that had to be put right, some of it urgently.
"The prison was experiencing real drift and performance was in decline. Some staff seemed overwhelmed by the challenges confronting them and needed more support."
Elsewhere, Nigel Newcomen, the Prisons and Probation Ombudsman (PPO), examined 80 out of 89 self-inflicted deaths of prisoners aged 18 to 24 between April 2007 and March 2014 for his most recent report.
The Ombudsman found prison staff frequently placed too much weight on judging how the prisoner seemed or 'presented', rather than on known risks, even when there had been recent acts of self-harm.
In one case, an inmate with a history of mental health problems and previous suicide attempts discovered his girlfriend had ended their relationship and, on the same afternoon, a close relative had died. D espite this, his level of risk was not reviewed and two days later he was found hanged in his cell.
The report also reveals a fifth - 20% - of 18 to 24-year-olds examined had experienced bullying in the month before their death, compared to 13% of other prisoners.
Mr Newcomen said: " Young adults can be a difficult and challenging, as well as potentially vulnerable, population to manage.
"In our sample of 80 cases of self-inflicted deaths going back to 2007, challenging behaviour was common, with prison records detailing warnings for poor behaviour, formal adjudications and punishments for breaches of prison rules."
The Ombudsman recommended prisons act more robustly to allegations of bullying, as well as more timely referrals for mental health treatment.
Labour's Prisons Crisis Summit will be attended by prison governors, prison officers, former senior officials, charities, voluntary groups, police and crime commissioners and local authority representatives.
Michael Spurr, chief executive of the National Offender Management Service (Noms), said: " Serco took immediate action in response to the inspection findings - strengthening the management team, prioritising safety and implementing a comprehensive improvement programme.
"I am confident that these actions have addressed the concerns identified by (the Chief Inspector of Prisons) but we will monitor progress closely to ensure the prison is able to deliver its regime safely and securely."
In response to the Ombudsman's report, Deborah Coles, co-director of Inquest, which provides specialist advice to people bereaved by a death in custody, said : "These deaths are the most extreme outcome of a system that fails some of society's most troubled and disadvantaged young people, many just out of childhood.
"The young people we incarcerate are some of the most vulnerable with histories of mental ill health, drug and alcohol problems, learning difficulties, abuse, and trauma.
"Today's report is yet more evidence of the fatal consequences of placing vulnerable young people in bleak and unsafe institutions ill-equipped to deal with their complex needs."
Commenting on excerpts of Mr Khan's speech on prisons, a Conservative spokesman said: "This is just political posturing from Sadiq Khan.
"What he won't tell you is that prisons are now less overcrowded, there is less self harm and the level of assaults is lower than under Labour. And those prisoners most likely to reoffend will now get a year's support when they leave.
"Of course there are additional pressures on prisons, because we have had to make realistic assessments to deal with Labour's record peacetime deficit.
"That means they are going through a period of change. But we are managing these pressures and our prisons are still running safe and decent regimes.
"This means we can make sure those who break the law are now more likely to go to prison, and go for longer than under Labour. That is part of our action plan to make Britain an even safer place to live, work, and raise a family."
Juliet Lyon, director of the Prison Reform Trust said: " On the day that the Prisons Ombudsman reports on vital lessons that must be learned if we are to prevent further young deaths in custody, we hear from the Chief Inspector of Prisons that Serco-run HMP Doncaster is a vast, filthy, drug-ridden institution with grossly inadequate first-night arrangements and poor staff supervision where violence and self-harm are rife.
"In dangerous environments like this the weak, people who are mentally ill, those with learning disabilities and vulnerable young people, suffer the most. In under two years our prison system has become less decent, less humane and less safe."