Prisoners being locked in their cells for 23 hours a day under Covid restrictions is dangerous, the chief inspector of prisons has warned.
Peter Clarke told BBC Newsnight that inmates were "losing hope" and warned of the impact on their mental health.
The Prison Officers' Association (POA) has said the practice - which aims to help reduce the spread of the virus - has reduced violence and self-harm.
It has led to a more stable environment, the POA said.
But Mr Clarke, who is due to step down from his position at the end of October, said he found that argument "shallow" and "to an extent self-serving".
In an in-depth interview with Newsnight, ahead of the publication of a report later on Tuesday, Mr Clarke said he had "good reason to believe" that self-harm was actually rising in women's prisons.
The inspectorate for England and Wales says it has visited more than 50 prisons since the beginning of the pandemic and found that prisoners spending 23 hours a day in their cells was "normal".
Sentenced inmates are allowed out of their cells to attend work or education, to exercise and to socialise for short periods. But since lockdown much of this activity has stopped.
"We cannot surely have a position where we say that the only way to run a safe and decent prison is to lock prisoners up in conditions that amount to solitary confinement," he said.
"That cannot be right. We as a nation surely cannot say that that is acceptable."
But the national chairman of the POA, Mark Fairhurst, said no-one on the front line "wished to see prisoners locked in cells for 23 hours a day".
"We must be mindful there are 20 outbreak sites at the moment, so Covid is a real and prevalent threat.
"Our duty of care is to people housed in those prisons, to keep them safe."
'So dirty you can't clean it'
Mr Clarke called on Justice Secretary Robert Buckland to work with the leadership in prisons and come up with a workable solution for however long the pandemic lasts.
In what was his final interview before stepping down, Mr Clarke also criticised prison service managers, saying they were out of touch with how bad conditions behind bars can be.
He says there has been an overall failure to manage the performance of prisons for too many years.
"I ask the question, why so often when the inspectorate has found really very disturbing and appalling conditions does it seem to come as a surprise to so many of the management of the prisons?
"I think the biggest problem is that there's been an overall failure to manage the performance of prisons for far too many years.
"The Prison Service prides itself on being what they call data driven and evidence based."
He urged managers to "taste it, to smell it" and to "see what is actually happening" in prisons.
Mr Clarke described some prisons as being allowed to get "so dirty you couldn't clean it", citing the example of one prison which had gullies full of rats and rubbish.
A Prison Service spokesperson said: "The measures we put in place limited the spread of the virus and saved lives.
"In line with the latest advice from Public Health England we are gradually easing restrictions, including increasing time out of cells."
Mr Clarke said he was hopeful for the future of prisons. "I am hopeful because before the pandemic struck, there were some signs that things were getting better."
He said some of his recommendations were being taken on board.
Mr Clarke's fifth and final report will be laid before Parliament today.
Charlie Taylor, a former chair of the Youth Justice Board, will succeed Mr Clarke as chief inspector.