There has been a substantial increase in violence in jails in England and Wales, the chief prisons inspector is expected to say in his annual report.
Nick Hardwick is also set to say prisons are failing in their objective to rehabilitate inmates.
It is his final report before leaving his post.
There were more than 15,000 assaults in men’s prisons in England and Wales last year, National Offenders Management Service data shows.
That is the highest annual figure in a decade.
‘Most damning’ report
Mr Hardwick, who has held the position for five years, is also expected to say there has been a growing number of attacks on prison staff.
The Today programme’s Sima Kotecha said the last annual report is set to be “his most damning yet”.
She said: “Nick Hardwick is expected to make reference to prisons such as Wormwood Scrubs and Nottingham – where inmates have reported feeling unsafe due to high levels of violence.
“Some of those working in the penal system argue there’s a danger that this kind of environment could turn inmates into even more dangerous criminals meaning they would be a bigger threat when they are released.”