Stress-related illness among prison staff has hit a five-year high following major cuts to the workforce, new figures have revealed.
Prison staff in England and Wales took off 54,519 days due to stress in 2014-15 - up from 53,290 the year before and 46,886 in 2012-13, figures obtained by the Press Association under Freedom of Information laws show.
The rise comes amid growing concerns at the state of Britain's prisons and is revealed just a month after staff staged a walkout at Wormwood Scrubs because they "don't feel safe".
Frances Crook, chief executive of The Howard League for Penal Reform, said the figures were just "the tip of the iceberg" and warned that the service is in crisis.
Between 2010 and 2011, 51,649 days were taken off due to stress by prison workers and in 2011-12, 50,051 were lost.
At HMP Downview, an all-women prison, each member of staff took an average of 4.7 days off due to stress in 2014-15 - the highest rate in the country.
Staff at HMP Leicester each had 4.3 days off with stress-related illness during this period, and workers at HMP Feltham had 4.1.
Ms Crook, who has been working with prisons for 30 years, said many employees were "reticent" to report stress-related illness and warned that rates could be much higher.
She said she had never seen jails "in such a bad state" and pointed to high suicide rates among prisoners - with around one inmate taking their own life every four days - as a source of trauma for workers.
"If you are a prison officer and you have to cut someone down who is hanging - imagine how stressful that is," she said.
"Prison officers save lives - they cut people down and save lives."
During the last five years the prison workforce has shrunk by almost 30%, from 46,090 members of staff in 2010-11 to 32,430 in 2014-15.
Ms Crook said a lack of staff and management in "overwhelmed" prisons had increased pressure on workers.
"They've cut out and got rid of a lot of prison officers, but they have also got rid of a lot of middle managers and governor grades," she said.
"So not only are you a prison officer along with around 150 prisoners, but you've got no management. There's so little management behind you, there's no-one to talk to or go and ask for help from, there's no supervision.
"You don't get that support, and you need it if you are in a very stressful job. You need a colleague or a manager.
"I'm not surprised you are seeing increased levels of stress - but there's a back story to it that is even more concerning."
In May, staff at Wormwood Scrubs prison walked out over health and safety concerns following a number of assaults on staff.
Mike Rolfe, of the Prison Officers' Association (POA), said at the time that the London jail was "flooded with drugs, mobile phones and weapons".
Official figures have also revealed soaring levels of violence and self-harm behind bars.
Data covering England and Wales shows there were a total of 100 apparent self-inflicted deaths in the year to March - the highest level for more than a decade.
There were nearly 5,000 attacks on staff in 2015, a jump of more than a third compared with the previous year.
Ms Crook said working in a prison had a "knock-on-effect" on mental, emotional and physical health.
She recently met a prison officer who had worked in a local jail who told her "he had spent his life shouting at his family" because "that's the way he behaved - he had to behave" at work.
The figures, which include staff in both operational and non-operational roles in public jails, showed "exactly why prisons are so badly in need of reform", a spokesman for the Prison Service said.
A package of reforms for Britain's prisons was announced in the Queen's Speech, including more autonomy for governors and plans to improve education.
Justice Secretary Michael Gove has also announced emergency funds for English and Welsh prisons to tackle violence and high suicide rates among prisoners.
The POA said it had criticised the National Offender Management Service (Noms) "year on year" for failing to recognise and support staff suffering from stress-related illness.
"The service has become more violent and as a result the working environment has deteriorated," a spokesman said.
"The changes to working arrangements has resulted in a return to the long hours culture which only makes staff more tired and vulnerable to stress-related illnesses. Staff are fearful of reporting stress symptoms for the fear of being dismissed.
"The real problem is that more than 60% of staff are continuing to work when they need professional support and counselling."
A spokesman for the Prison Service said: "These figures show exactly why prisons are so badly in need of reform. Dedicated prison staff work in an extremely challenging environment in which, on a daily basis, they face unique circumstances unlike any most others in the public sector.
"We are investing £1.3 billion to transform the prison estate over the next five years to better support rehabilitation and tackle bullying, violence and drugs.
"The safety, welfare and well-being of our staff is a top priority and we will always ensure prisons have enough staff to run safely and securely."
He added that 2,830 prison officers had been recruited since January last year in response to staffing pressures, a net increase of 530 officers.