Frontline prison officer numbers have fallen amid soaring levels of violence behind bars, according to campaigners.
The Howard League for Penal Reform said staffing figures showed a reduction over the last year.
Statistics show there were the equivalent of 14,689 full-time frontline officers in England and Wales in June, according to the charity - a fall of just over 400 compared to a year earlier.
The figures are for staff bands which cover personnel whose roles include working directly with inmates and supervising activity.
Andrew Neilson, of the Howard League, said: "Reducing resources while allowing the prison population to grow unchecked has created a toxic cocktail of violence, death and human misery.
"These figures show how reductions in staffing and problems in recruiting and retaining new staff are feeding the problems behind bars."
Concerns have repeatedly been raised about safety in jails in recent months.
In the year to the end of March there were 5,423 assaults on officers - up 40% on the previous year. The tally included 646 serious incidents.
A Ministry of Justice spokeswoman said: "Safe prisons are fundamental to the proper functioning of our justice system.
"Our dedicated prison staff, who support tens of thousands of prisoners every day, are vital to the safe running of our prisons.
"We have recruited 2,900 staff over the last 12 months and are taking significant action to make sure we have appropriate staffing levels.
"The Secretary of State is determined to make sure our prisons are safe and places of rehabilitation and will set out her plans for reform shortly."