Prisoners are being forced to sleep in toilets due to overcrowding.
The' report into conditions at Doncaster Prison, South Yorkshire, found that two-person cells had been turned into three-person cells by placing a bed in the shared toilet.
The unannounced inspection also found a "worrying" deterioration in healthcare for prisoners, which needed to be urgently addressed, and an increase in violence and self-harm.
The inspection, in February, found the category B prison had made progress in some areas but there were still some outstanding concerns.
The report highlighted several problem areas, which included the addition of an extra bed in the toilets of two-person cells, poor access to a GP or dentist for prisoners, and frailties in anti-bullying and suicide prevention arrangements leading to an increase in incidents of violence and self-harm.
It also stated that little progress had been made in expanding the very limited amount of purposeful activity and some prisoners at risk of self-harm were being inappropriately held in a healthcare dormitory.
The inspection did find an improvement in relations between prisoners and staff, better management of prisoners' early days in custody and that an earlier recommendation for a first-night centre had been implemented.
The jail, which has a capacity of 1,145, was also commended for its resettlement provision, which was described as among the best in the country for a busy local prison.
Commenting on the report, chief inspector of prisons Anne Owers said: "Doncaster has addressed a number of the criticisms that we made on our last visit and continued to make good progress in areas such as resettlement.
"However, there is much still to do, not least to ensure that safety is maintained and the serious deficits in healthcare urgently addressed. There has also been a disappointing lack of progress in responding to our previous concerns about the inadequate quantity and quality of purposeful activity."
