A prison governor was seriously injured after he was beaten by an inmate in an "unprovoked" attack in the canteen.
Paul Cawkwell was talking to a prisoner in the canteen at HMP Wayland in Norfolk when the beating took place, a source told BBC News.
The Ministry of Justice confirmed to ITV News that they were investigating the assault.
A spokesperson 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."
The attack, said to be an isolated incident, took place on July 27.
Concerns about assaults on staff were raised in the Category 3 prison's last annual report by the Independent Monitoring Board.
It said that assaults on staff had increased and expressed concerns about budget cuts and "how they impinge on staff morale... and how a safe environment can be maintained".
But it commended the "strong management led by the Governor and his team".
In July, Wayland inmate Daniel Beckford, 34, was jailed for 20 months after assaulting two prison officers.
He was already serving a five year sentence for burglary and assault when he assaulted the officers during a roll call in November 2015.
One was bruised in the attack while the other needed treatment for a deep bite wound to the chest.
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.