Police are investigating an attack on a prison governor by an inmate.
Paul Cawkwell, governor of HMP Wayland in Norfolk, was targeted by a prisoner in the canteen of the Category C men's jail.
He reportedly needed hospital treatment for his injuries.
A Prison Service spokesman said: "We do not tolerate any violence against our hard-working and dedicated prison staff.
"Safety in prisons is fundamental to the proper functioning of our justice system.
"When incidents like this occur we work closely with the police to push for the strongest possible punishment.
"This incident is now being investigated by police, therefore we cannot comment further at this stage."
In its last annual report, the prison's independent monitoring board had voiced concerns about staffing levels.
But sources said the assault on the governor was an isolated incident which involved one prisoner and had nothing to do with staffing levels.
In May around 20 prisoners were involved in a food fight at the jail and in June an inmate attempted to grab a set of keys from an officer.
HMP Wayland opened in 1985 and has an operational capacity of more than 1,000.