Almost 20,000 prisoners were kept in overcrowded cells last year, figures obtained by a charity have revealed.
About 19,140 prisoners on average were forced to share a cell designed for one person during the financial year 2012-13, the Howard League for Penal Reform has said.
A further 777 people were made to sleep three to a cell, when the cells are designed to accommodate only two.
Official government prison population announcements mask the full extent of overcrowding because they do not state how many cells are holding more prisoners than they are designed to.
The worst-affected prison in England and Wales was Wandsworth where, on a typical day, 835 prisoners were forced to share cells which contain an open toilet, the Howard League said. Other overcrowded prisons were Altcourse, Doncaster, Birmingham, Pentonville, Preston, Manchester, Elmley, Nottingham and Durham.
Frances Crook, Howard League chief executive, said: "At last, we have the picture of the real state of overcrowding in our prisons. It's far worse than anyone imagined: one in four people behind bars are packed like sardines into cramped cells.
"It should come as little surprise that such crowded conditions leave staff hugely overstretched, especially as more are being laid off. This means there are little-to-no opportunities for prisoners to work, learn or take courses to turn them away from crime.
"Staff cuts and overcrowding mean that grown men spend all weekend and up to 22 hours a day during the week cooped up like battery chickens. No wonder violence and self-injury is rife. If the Ministry of Justice is serious about reducing reoffending it must tackle overcrowding now. Successive governments have peddled the lie that you can build your way out of a prisons overcrowding problem."
Prisons Minister Jeremy Wright said: "Let's be clear what overcrowding in prison actually means - typically it means having to share a cell rather than have one to yourself. 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. We are replacing older prisons with newer accommodation that is cheaper to run. I will continue to look for ways to make the prison system more efficient and to tackle our stubbornly high reoffending rates."