A prison officer has been injured after a riot at a jail in Rutland involving around 60 inmates, the Prison Officers Association said.
The "serious incident of indiscipline" occurred at HMP Stocken after one prisoner assaulted an officer on 14 June, causing others to become "disorderly".
Police and fire crews had to be called after several small fires were ignited during the disturbance. The incident was resolved around six hours after the disorder broke out when the riot squad were called.
All 120 prisoners on the wing at the Category C prison were moved away from the area, with around 30 inmates transferred to other high-security prisons.
Four prisoners have been taken to hospital and two prison officers have been treated for smoke inhalation following the riot. One officer who was assaulted was treated at hospital and later discharged.
A Prison Service spokesperson said: "A serious incident of indiscipline on one wing at HMP Stocken was resolved by specially-trained prison officers.
"This kind of behaviour is totally unacceptable. We are absolutely clear that prisoners who behave in this way will be punished and can spend significantly longer behind bars. Those suspected of being actively involved in this incident have already been moved to higher security prisons."
A Prison Officers Association spokesperson said: "The Union believes had it not been for the professionalism of staff then this disturbance could have been far worse.
"The Union awaits the outcome of an investigation into what started this disturbance which has resulted in fires being lit, an injury to an officer and prisoners being transferred out with substantial damage to the wing.
"The POA have consistently stated that cuts have consequences and incidents are on the increase."