Prison reform campaigners have called for urgent action at Britain's news private jail, which has been put into "lock-down" in response to the violent gang warfare behind its bars.
In an inspection report, Serco-run Thameside prison in London was found to keep some prisoners in their cells 23 hours a day, with 60% locked up all day, in order for staff to cope with the violent offenders.
The Howard League for Penal Reform called the report "truly alarming" and said it was an embarrassment to the government.
The previous system, where prisoners were allowed out of their cells for most of the day, had seen an "unacceptable" level of delinquency, the prison said.
The report by HM Inspectorate of Prisons found that the prison, which opened in March last year holding 900 male prisoners, had far too high a frequency of assault, and that staff could not protect prisoners from the violence.
"The prison had taken the unusual step of effectively locking down the prison, severely curtailing the regime and in particular prisoner access to time unlocked.
"The prison had done little to evaluate the success of this quite extreme strategy and at the time of our visit there seemed only vague plans to restore the prison to normality.
"The data on assaults, security report reports and use of force that we examined did not show any improvement from previous months and we were told that some prisoners got around restrictions by planning to attend activities so that they could become involved in fights."
Andrew Neilson, Director of Campaigns at the Howard League for Penal Reform, said in a statement: “Conditions at Thameside are truly alarming. Violence was so common that the Serco management put the prison in a state of lockdown, and yet this extreme measure has done little to bring down the number of attacks.
“Staff are inexperienced and often resort to physical force. The prisoners have no confidence in them. Despite enforcing one of the most restricted regimes ever seen by inspectors, this is a large private prison out of control.
“Today’s report should embarrass the government. Less than a week after Justice Secretary Chris Grayling demanded that prisoners work harder to earn privileges, this flagship private prison is revealed to be locking up inmates for 23 hours a day because they don’t have anything constructive to do.
“With a pathetic lack of activities and barely any vocational training available, Thameside is doing nothing to help prisoners turn their lives around. It is merely making matters worse.
“This is what happens when you hand the justice system over to vast multinational corporations, who put cost-cutting and the interests of their shareholders ahead of concern for public safety.”
The National Offender Management Service said action had been taken to address the concerns.