Three prison officers have been stabbed by an inmate at the high- security Frankland prison in County Durham.
Two of the victims, a man and woman, have undergone surgery; a second male officer suffered less serious wounds. The officers were allegedly attacked with a broken bottle on Saturday morning.
The officers are believed to have been assaulted as they opened a cell door at the Category A prison. The female officer was stabbed in the back and her two male colleagues in the face and side.
Colin Moses, chairman of the Prison Officers Association, said: "This incident shows the level of violence our officers face on a daily basis at Frankland and other prisons."
Mr Moses said there was a level of violence "deemed acceptable" within prisons, and has called for a zero-tolerance approach.
A spokesman for the Ministry of Justice said police were investigating.
He added: "Violence in prisons is not tolerated in any form. Assaults on prison staff are taken very seriously and depending on the severity will be dealt with internally or referred to the police."
A police spokesman added: "This was a very, very serious attack and but for the grace of God we could have been looking at murder here. The public will want to know why prison officers are not provided with stab-proof vests and other equipment."
Prison inspectors say Frankland holds some "extremely challenging prisoners" such as those with affiliations to gangs or a history of extreme violence. It also houses prisoners convicted of serious sexual offences, some with severe personality disorders, and convicted terrorists.
The prison has been at the centre of several high profile incidents in recent years. In 2008 the HM Inspectorate of Prisons raised concerns about escalating violence.
Terrorist Dhiren Barot suffered burns in 2007 when another inmate poured boiling liquid over him. A convicted armed robber was injured in a similar attack months later.
In 2008 trouble broke out when the cell of another terrorist prisoner, Kamel Bourgass, was set ablaze. Weeks later a prison officer was injured during a riot when nine prisoners smashed up their cells.Reuse content