The number of officers in public sector prisons has been cut by 41% in less than four years, according to new figures obtained by penal reformers.
The Howard League for Penal Reform used official Ministry of Justice (MoJ) figures to show the scale of the cuts hitting the prison service, amid a series of reports warning of a crisis across the estate.
"The prison system is in crisis, and these figures reveal why. While the prison population has grown, officer numbers have been cut without any thought for the consequences," chief executive Frances Crook said.
There were only 14,170 officer grade staff working in public sector prisons in June 2014, compared to over 24,000 at the end of August 2010.
Prisons such as Kennet, Onley, Askham Grange and Ranby have cut more than half their officers.
Prison reformers warn that staff shortages increase the risk of disorder, lead to inmates spending 23 hours a day in their cells and deprive them of access to facilities like the library or the gym, because there is no-one to escort them.
They also warn it has fundamentally broken the system of officer care in prisons, with the decline in officer numbers coinciding with a soaring rate of suicide and self-harm behind bars.
This year alone, there have been 54 suicides in prison.
A damning series of reports from Her Majesty's Inspectorate of Prisons have warned of safety issues in Ranby, Glen Parva, Hindley, Isis, Wormwood Scrubs, and Swaleside prisons.
Last week, the outgoing president of the Prison Governors' Association revealed officers were being sent south to fill in gaps in the system while staying at £500-a-week hotels – an arrangement the Howard League believes is part of long-term planning.
"As well as being a shameful waste of taxpayers' money, this approach will only create more disruption in jails," Crook said.
"Good relationships between prisoners and staff are key to a well-run prison, and such relationships will be harder to achieve.
"Prison officers must respond to emergencies, and it is potentially disastrous to ask lowly-paid staff, demoralised and far from home, to work in different, unfamiliar prisons each week."
The MoJ disputed the Howard League figures and said the actual reduction in prison officer numbers was 27%.