Three prison officers have been rushed to hospital after Hashem Abedi, the brother of the Manchester Arena bomber, "threw scalding oil and stabbed them", a union said.
Abedi, who is being held at HMP Frankland in County Durham for his role in the deadly attack eight years ago, inflicted "life-threatening" injuries on the three officers on Saturday, according to the Prison Officers Association (POA).
The union said that he had used home-made weapons to stab them. At least one of the victims was a female police officer.
The ambulance service confirmed that three people had been taken to hospital. Police said that two of the victims still are in a serious condition, while one has been discharged.
The officers are thought to have sustained burns and stab wounds in the attack, after Abedi threw hot cooking oil over them, before using makeshift weapons to attack and stab prison guards, the Prison Officers Association union said.
It's reported that one officer was stabbed in the face and throat, later being airlifted out of the prison for emergency surgery at the James Cook University Hospital in Middlesbrough.
Henry Riley reports from what will become the UK's largest prison
A second officer injured in the attack is still being treated in hospital, with a third female officer released following hospital treatment.
Secretary of State for Justice, Shabana Mahmood, said: "I am appalled by the attack of three brave officers at HMP Frankland today. My thoughts are with them and their families.
"The police are now investigating. I will be pushing for the strongest possible punishment. Violence against our staff will never be tolerated."
The national chairman of the POA, Mark Fairhurst, said: “First and foremost, my thoughts are with the injured staff, their families and colleagues. No Officer should be subject to cowardly and vicious attacks at work.
"The POA will support our members as much as we need during this traumatic time, this attack displays the dangers brave Prison Officers face on a daily basis.
"Separation Centres hold the most dangerous terrorist offenders who simply do not wish to alter their ideology and as this event confirms, are determined to inflict violence on those who hold them securely.
"We must now review the freedoms we allow separation centre prisoners have. I am of the opinion that allowing access to cooking facilities and items that can threaten the lives of staff should be removed immediately."
"These prisoners need only receive their basic entitlements and we should concentrate on control and containment instead of attempting to appease them. Things have to change.”
A Prison Service spokesperson said: "Three prison officers have been treated in hospital after an attack by a prisoner at HMP Frankland. Police are now investigating so it would be inappropriate to comment further. Violence in prison will not be tolerated, and we will always push for the strongest punishment for attacks on our hardworking staff."
A spokesperson for North East Ambulance Service said: "We were called at 10.50h today (12 April) to attend an incident at HM Prison Frankland in County Durham.
"We dispatched two paramedic ambulance crews, a doctor in a rapid response vehicle, the air ambulance with a second doctor, a specialist paramedic, a Harzardous Area Response Team (HART) crew and a duty officer.
"We treated three patients before conveying all to hospital by road. Two of the patients were accompanied by a doctor."
A Durham Constabulary spokesman said: “An investigation is underway following a serious assault which occurred at Frankland prison, Durham today.
“Three victims were taken to hospital to be treated where two remain with serious injuries and a third has been discharged.
“Enquiries are now ongoing.”
Abedi helped plan the Manchester Arena bombing, which was carried out by his brother Salman.
Some 22 people died, and over 1,000 were injured.
Hashem Abedi was jailed for life with a minimum of 55 years. Salman Abedi died in the attack.
HMP Frankland, nicknamed 'Monster Mansion', houses several of the UK's most high-profile criminals.
Inmates include Wayne Couzens, the former police officer who murdered Sarah Everard, serial killer Levi Bellfield, a and Urfan Sharif, who murdered his daughter Sara.
