Around 85% of prisoners were kept in their cells for 23 hours a day for the majority of the COVID lockdown period - with more than a third showing symptoms of severe anxiety disorder, research has indicated.
The report Coping with COVID in Prisons found measures introduced in response to the pandemic led to periods of prolonged solitary confinement across the prison population.
It said this resulted in dramatically increased levels of anxiety and depression.
The study, funded by the Economic and Social Research Council, was a partnership between the ex-offender-led charity User Voice and social scientists at Queen's University in Belfast.
User Voice's founder and CEO, Mark Johnson, said: "The report reveals one of the darkest and most hidden results of the pandemic, the true effects of extreme lockdown and confinement on prisoners and ultimately, on the public.
"It shows that we need to talk about criminal justice. Are prisons just for punishment or are they failing prisoners and the public if they don't offer the support which leads to rehabilitation?"
Professor Shadd Maruna, Professor of Criminology at Queen's University Belfast, said: "Our research definitively demonstrates that the social climate in prison has become dramatically worse after the lockdown, and a great deal of work is going to be needed to restore a sense of trust and legitimacy among the incarcerated.
"Peer-led models, like the kind that drove this research project, have the potential to do just that if implemented correctly."
A Prison Service spokesperson said: "Our tough but necessary action during the pandemic saved the lives of many staff and prisoners - and we quickly rolled out measures such as video calls and in-cell education in recognition of the impact.
"We continue to increase mental health support and improve training for staff, and our Prisons Strategy sets a clear vision to provide all offenders with the education, skills and support they need to get back on the straight and narrow."
Nearly 100 serving prisoners were trained to survey fellow inmates during the research.
Over the 18-month project, these volunteers completed more than 1,400 surveys with fellow prisoners across 11 prisons in England and Wales.
The prisons which participated were given institutional anonymity.
Key findings of the report included:
• 85% of prisoners surveyed were confined to cells for 23 hours for most of the lockdown period
• 59% of prisoners surveyed had not had a single visit with family during the COVID lockdown
• Standard screening tools suggest depression and anxiety scores are almost five times higher than the standard for the general population
• More than one out of three prisoners were scoring at the level of "severe anxiety disorder"
• Two thirds of survey respondents said access to mental health support had worsened