An independent report on Ranby Prison in Notts has called for better accommodation and chaplaincy service.
Ranby Prison has about 1,000 prisoners
The Independent Monitoring Board wants the prison's oldest wing replaced, but the governor has said that option is "too expensive".
The prison had a £27m expansion last year to bring its capacity to more than 1,000 inmates.
The report also raised concerns about morale among health care staff and a shortage of chaplains.
Former army camp
But Ranby governor Phil Wragg said: "We have achieved fantastic results in the past year and have reached an 'establishment of excellence' status within the Prison Service.
"Staff have turned around a failing establishment and turned it into a beacon of excellence."
Mr Wragg said new management practices have been introduced at the health care centre and waiting lists have been reduced by half, compared to last year.
He also said the prison was working hard to recruit another chaplain due to retirement.
Carole Gee, chairwoman of the monitoring board, said: "The number of prisoners at Ranby has increased, but the number of medical and chaplaincy staff hasn't gone up."
She praised the staff for coping with the expansion at the prison but said the lack of toilets in some cells was not acceptable.
The category-C prison was a former World War II army camp - and was turned into a prison in the early 1970s.