Two prison officers have been taken hostage and attacked by three inmates at a maximum-security jail near York.
The incident on Sunday at Full Sutton Prison in East Yorkshire lasted for four hours.
The Prison Service said the staff were treated for injuries which were not thought to be life-threatening.
The Prison Officer’s Association (POA) said it was aware of the “hostage incident”. The North East Counter Terrorism Unit is investigating.
The POA said it was sending a national representative to the prison to determine exactly what happened.
Steve Gillan, the POA’s general secretary, said: “Until the full facts of the incident are known we do not wish to comment further for fear of compromising any police investigation.
“We can confirm that officers sustained injuries and had it not been for the professionalism of prison officers dealing with this violent incident the outcome could have been worse.”
The Prison Service spokeswoman said the incident started at 16:25 BST and ended at 20:40 “after staff intervened”.
She would not confirm reports that one of the prison officers was held hostage and stabbed, or give details of the identities of the prisoners involved.
Mark Leech editor of Converse the national newspaper for prisoners said the High Security Estate was currently on ‘tenterhooks’
“Since the savage and brutal murder of Drummer Lee Rigby the whole of our High Security Prison Estate has been on tenterhooks, prison staff have been advised to be extra vigilant for anti-muslim tensions or pre-emptive attacks by muslim inmates who fear for their safety in the aftermath of the soldier’s murder.
“Our High Security prisons are extremely difficult to manage at the best of times, in the current climate they become even more so and its a tribute to the Tornado Team who made the intervention that no serious injuries were sustained.”