A prison officer was left unconscious after being attacked by an inmate.
The officer was assaulted overnight at HMP Lindholme, near Doncaster, and later went to hospital, the Ministry of Justice (MoJ) confirmed.
Prison staff initially refused to go back on to the wings following the attack and staged a protest in the visitors centre.
The prisoner involved had been segregated, an MoJ spokesman said. Full details of the attack are not yet known but the MoJ confirmed the officer was found unconscious and treated at the prison before receiving hospital treatment.
An investigation had started, it added.
Staff at the Category C jail went back to work following the protest and a meeting with the governor over safety concerns.