A prison governor has been seriously injured in an "unprovoked" beating at the hands of an inmate.
Paul Cawkwell was talking to a prisoner in the canteen at HMP Wayland, Norfolk, when he was attacked, a source told the BBC.
Police said an inmate was transferred to another jail after last Wednesday's attack.
The Ministry of Justice confirmed that police were investigating the assault.
The source expressed concerns about staffing at the category C prison, echoing the findings of a report last year.
"Out of anyone, you would have thought there would have been people around who would have been able to get him away quickly."
Mr Cawkwell, who has held a number of senior positions in the prison service, has received hospital treatment for his injuries.
A prison service spokesman said it did not tolerate violence against staff and would "push for the strongest possible punishment".
Alex Hewson, policy officer from the Prison Reform Trust, said: "Unfortunately it's part of a growing problem in our prisons.
"Over the last five years we've seen people in prison, that's prisoners and staff, less safe, and with higher levels of assaults, higher levels of self-harm and deaths as well.
"It's a deeply troubling time for prisons at the moment."
In its last annual report, Wayland's Independent Monitoring Board said it was concerned about staffing at the prison citing "staff shortages of up to 22%".
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Vice chairman Roger Marston said the board was due to publish its latest report soon and that "a huge amount had been done in terms of recruitment and we have a full complement of staff."
He said that although not huge, there had been an increase in violence at the jail this year.
There have been a number of reports of disruptions at HMP Wayland this year.
In May, about 20 inmates were involved in a "food fight" which broke out during lunch, while two inmates attempted to grab keys from officers in a separate incident in June.
Another HMP Wayland inmate was jailed in June for an assault on two guards, during which one was bitten, according to police reports the BBC.