Daniel can’t talk about the three prison inmates who have died in custody over the last five years - two of the victims for legal reasons, the other because it was too “horrendous”.

The prison officer, who spoke to The Huffington Post UK on the condition of anonymity, says: “Two of them are subject to corner's court. I can't talk about the one that's not. I don't want to talk about it because it was horrendous.”

He adds: “Up-and-down the country we've had a fair share of deaths of custody.”

A fair share, to be precise, is 44 so far this year and 193 in 2011, according to figures from the Howard League for penal reform.

In the larger prisons, the numbers are worse. Eighty-two people died in the 25 largest prisons in Britain last year, prompting the Howard League to warn of the damage of overcrowding.

For Frances Crook, chief executive of the Howard League, it’s simple: people should not be dying in the custody of the state.

“Prison is quite a toxic environment - don't forget it most resembles battery chickens,” she tells The Huffington Post UK. “People are cooped-up; they have a restricted diet and no natural light. If you are kept like that for several years it shortens your life.

“You get people coming into prison who may not have had the healthiest lives. That's why you get people dying from natural causes in their 40s and 50s.”

While prisons minister Crispin Blunt has said the government is “fully committed to reducing deaths in custody” there are concerns that conditions in prisons are worsening.

The Howard League isn’t the only organisation that believes there is a problem. On Tuesday a coalition of more than 60 charities called for action on prison over-crowding, pointing out that a report by the Joint Committee on Human Rights in 2004 raised concerns about the link between prison overcrowding and suicides in custody.

“The disruption of churn can do damage to a prisoner’s mental wellbeing; 26% of self-harm incidents occurred within the first month of arriving in a prison,” the report from the Criminal Justice Alliance warned.

“The Prison and Probation Ombudsman found in an analysis of deaths in prison that the location of the cell was inappropriate in 10% of all cases.”

For Crook the problem is clear: “Prisons are frightening and people kill themselves, between one and two a week. Anyone who says it's a holiday camp, it's just not true. It's boring, it's frightening, you're caged and also they are people who are very vulnerable,” she says.

“It's absolutely getting worse with overcrowding and over-use of prisons... We're sending too many people to prison who don't need to be there, and it creates a sense of injustice.”

It’s not just the inmates that find prison frightening. Ralph Valerio, a member of the national executive of the Prison Officers’ Association, says officers are scared too - and often outnumbered by prisoners they don’t know how to look after.

“There’s a lot of fear in some establishments. Prisoners are scared. Staff are scared. Prison is a scary place,” he tells The Huffington Post UK.

“There are many people who are in prison who the prison system is just quite simply not set up to look after.

“I’ve known lots of situations with men banging their heads on a wall, bleeding, blood coming out of their heads and their noses, scarring themselves. Then mental health people come, have a look at them and say ‘nah, they’re not mad, they’re bad.’ I’m sorry, I don’t understand that.”

As for overcrowding, Valerio claims that “the prison system cannot carry on as it is and even pretend to be serving the public.”

“Overcrowding has only one direct effect, in the respect that it decreases the standard of living for the people that are in the prison and of course if you have a reduction in the delivery of quality, as a result of overcrowding then the end result is a disgruntled prisoner, absolutely no rehabilitation, and a more violent population.”

Currently the prison population is 87,787, rising at a rate of 2.9% compared to last year, with some prisons at nearly twice their official capacity.

According to Daniel, who has more than 18 years of experience in the job, “every prison in the country is overcrowded.”

“It can be very scary. If they wanted to take a prison they'd take a prison, no problem,” he says. “Without a shadow of a doubt it has got worse recently. You can't rehabilitate people if you've got no one there [to help them]. The probation service, police, us, we're all being cut, seriously cut.

“Do we lock up people with psychosis and schizophrenia and all that? Yes we do. The prison system will lock up the worse of the worst.”

Daniel believes government cuts are only increasing the problems: “At the moment our hands are tied and we’re just waiting for the next cut.

“Self harm is an inherent problem within prison. There is not the required amount of beds in institutions that can look after the mentally ill and the courts will put them into prison. My job is about public protection. By locking those people up the public are protected.”

For many overcrowding is just a word. For Daniel, it’s a cell inhabited by three people who sleep, eat and go to the toilet in one room.

“Triple cells? Are they inhumane? Yes. They're just horrible. You've got three grown men in a cell, what size are we talking about - 12 by 8(ft)? Two in bunk beds, one in a single bed. It's a nightmare scenario. You've got a toilet that's detached, it's horrible. The honest truth, it's degrading. Are we going to do anything about it? No.”

For the Howard League the problem with overcrowding is clear; Prisons are inherently more dangerous, the situation becomes more terrifying, and rehabilitation goes out of the window, with prison staff just focusing on managing numbers and preventing riots.

And they’re finding alliances with individual officers, and some in the POA who are calling for change. For Valerio, "money dictates much more than the health and safety of everyone that lives and works in a prison.

“It actually says on every prison officer’s warrant card - as you would expect police officers to have a warrant card we have one too - it says our purpose is to serve and protect the public by leading those in our custody to have more law abiding and useful lives. That just doesn’t happen. It just doesn’t happen.

"They don’t get encouraged to lead useful and law-abiding lives because we are not actually encouraged to encourage them so it’s sort of contradictory even from the outset of what the primary objective of the prison system is.”