Two prison officers have been taken to hospital following an assault at Wormwood Scrubs prison.
The attack took place at the west London jail just one day after staff returned to work following a demonstration over health and safety concerns.
They were taken to hospital for treatment and the attacker was placed in segregation, the Ministry of Justice confirmed.
A police investigation will be launched and the general secretary of the Prison Officers' Association (POA) said there was "clearly an issue" at the facility which needed to be addressed.
A Ministry of Justice spokeswoman said: "Following an incident at HMP Wormwood Scrubs on Sunday May 8 two members of staff were taken to hospital for treatment.
"We do not tolerate violence against our hard-working officers. The perpetrator has been placed in segregation and will face a police investigation which could lead to criminal charges."
A number of officers gathered outside the prison on Friday to raise concerns over the number of assaults by inmates in recent months.
The temporary walk-out came weeks after an inspection report sparked claims of Dickensian squalor at the jail - one of the most famous in Britain.
POA general secretary Steve Gillan said: "We know there was an assault on two prison officers that involved hospital treatment.
"Clearly there is a growing issue there and we hope we can work with the local management to resolve it."