Overcrowding is rife at Westcountry prisons where more than 500 prisoners are crammed into cells designed for fewer inmates than they are currently holding.
The union representing prison guards condemned the situation as a "recipe for disaster" and fears widespread disorder will break out.
John Hancock, who represents the South West on the national executive committee, of the Prison Officers' Association, said the problem could not be ignored.
"We have been saying for many months that prison closures, reducing staff and overcrowding is a recipe for disaster," he said.
According to figures compiled by the Howard League for Penal Reform, about 19,140 prisoners on average were forced to share a cell designed for one person during the financial year 2012-13.
In the Westcountry's three prisons overcrowding remains a reality of daily life, the figures reveal.
At Exeter jail, there were 416 prisoners sharing overcrowded cells designed for just one person.
At Channings Wood, near Newton Abbot, 71 prisoners shared cells designed for lone occupants.
Meanwhile at Dartmoor, 27 prisoners were crammed into cells which in theory should only have held one person.
The Government denied overcrowding was an issue and said the nation's prisons had "safe population levels".
But Mr Hancock was sceptical: "They say no prison is overcrowded but when I have gone into prisons in the South West they have been overcrowded."
He said it was not simply an issue of comfort for an inmate.
"When you have overcrowding it causes anxiety among inmates and suicidal thoughts are often progressed because they are crammed into their cells," he said.
"When there is overcrowding in any prison it is more difficult to ensure that everyone gets what they are entitled to, such as their exercise or attending a church service.
"If a prison becomes overcrowded it is simply more difficult to operate."
Frances Crook, Howard League chief executive, said the charity had obtained a true picture of the real state of overcrowding in prisons.
"It's far worse than anyone imagined: one in four people behind bars are packed like sardines into cramped cells," she said.
"It should come as little surprise that such crowded conditions leave staff hugely overstretched, especially as more are being laid off."
Prisons Minister Jeremy Wright rejected the criticisms: "Prisoners are treated humanely but prison is not somewhere that anyone should be comfortable about going back to.
"All prisons have safe population levels and have capacity to take those sent there by the courts."