Prison staff had to receive medical treatment after violence broke out at a maximum security jail.
A "small number" of prison officers were hurt at HMP Whitemoor, close to March, Cambridgeshire, late on Monday afternoon, a prison service spokesman said.
Trouble flared on one wing and was "successfully resolved", he confirmed.
The BBC believes six staff members were taken to hospital after a disturbance involving about 40 prisoners.
An investigation into the violence at the category A and B prison will take place.
"We do not tolerate violence in our prisons and, where incidents like this occur, will always push for the strongest possible punishment for those involved," the spokesman said.
More than 400 men are housed in the three-wing prison, including a number of the highest-risk inmates.