Prisoners are having to sleep in the toilet area of their cells to ease overcrowding at a jail in northern England, a watchdog said in a report on Tuesday.

Chief Inspector of Prisons Anne Owers said cells designed for two inmates had been turned into three-man rooms by putting a bed in the shared toilet.

Inspectors uncovered the practice during an unannounced visit to Doncaster Prison in February. The jail has nearly 1,000 inmates, 200 more than its normal capacity.

"We were disappointed to find that two-person cells had been turned into three-person cells by placing a bed in the shared toilet," Owers said in the report. "This was unacceptable."

Prison governors are having to cope with record numbers of inmates. Earlier this year, Justice Secretary Jack Straw urged magistrates to jail fewer criminals to ease overcrowding.

The Howard League for Penal Reform said the report highlighted problems that affect prisons across the country.

"In such an atmosphere, it is no wonder that self-injury and suicide is on the rise at Doncaster," the charity's director Frances Crook said. "The violence and poor conditions put us all at risk as the overwhelming majority of the prisoners released from Doncaster will go out and commit more crimes."

The Doncaster report also raised concerns about healthcare standards. Access to a GP or dentist was described as poor. Incidents of violence and self-harming had risen.

Inspectors said they were disappointed by the lack of work, training and other purposeful activities for prisoners.

However, the inspectors noted some improvements since their last visit in 2005 when they found squalid accommodation and a lack of provision for new inmates.

They praised the relations between staff and prisoners and the standard of the food, shop and legal advice.

Michael Spurr, chief operating officer for the National Offender Management Service, said work had begun to improve the healthcare and activities at the jail.

"There remain areas for improvement and these will be monitored closely," he said.