A report has revealed that 366 prisoners at HMP Bullingdon were last year forced to share cells designed to accommodate only one person.

The Howard League for Penal Reform said that the research showed that prison overcrowding was a greater problem than ministers had previously suggested.

It said the average number of prisoners to be forced to share an overcrowded cell at the category-C prison was 366 in the financial year from 2012-13.

The figures released by the Howard League show that nationally 19,140 prisoners on average were forced to share a cell designed for only one person.

A further 777 prisoners were made to sleep three to a cell, when the cells were designed to accommodate only two.

Howard League chief executive Frances Crook said: “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. Staff cuts and overcrowding mean that grown men spend all weekend and up to 22 hours a day cooped up like battery chickens – no wonder violence and self-injury is rife.”

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.”

The facility came under fire in January when concerns were raised about “bullying” by a small number of staff at the prison following a report by the Independent Monitoring Board.