The chief inspector of prisons has said it is "unacceptable" that some prisoners at a Worcestershire jail still use a bucket in their cells rather than a toilet.
His report on HMP Long Lartin said four wings should be refurbished to include integral sanitation.
The National Offender Management Service said prisoners in the jail did not routinely have to slop out.
It denied that sanitation arrangements at the jail were unacceptable.
The report said the vulnerable prisoner wings at Long Lartin had a system where prisoners could ring a bell at night and leave their cell one at a time to use the toilet.
It said "inevitably" the system did not work effectively and prisoners often had to use a bucket in their cell which they emptied in the morning.
Nick Hardwick, the Chief Inspector of Prisons, said: "Whatever the official title, this was slopping out."
Michael Spurr, Chief Executive of the National Offender Management Service, said: "There is more to do and the governor and his staff will continue to work on the areas highlighted for improvement in the report."
The National Offender Management Service is part of the the Ministry of Justice and is responsible for prisons and prisoners in England and Wales.