Two prisoners have been arrested following a stabbing which left one inmate dead and two others in critical condition.
The Metropolitan Police have launched a murder investigation into the attack at Pentonville Prison, north London, at around 3.30pm on Tuesday.
Officers and paramedics from London Ambulance Service found the three prisoners with stab wounds and one of the inmates, a man in his 20s, was pronounced dead at the scene at 4.25pm.
The two other men, aged 21 and 30, have been taken to an east London hospital where they are in a critical condition, a Met spokesman said.
On Tuesday night a 34-year-old man was arrested and taken to a north London police station for questioning and on Wednesday morning it was announced a second prisoner, 26, had also been arrested.
A spokesman said: "Detectives from the Homicide and Major Crime Command are investigating. Further enquiries into the incident continue."
A Prison Service spokesman said: "Police are investigating an incident at HMP Pentonville this afternoon. It would be inappropriate to comment further at this stage."
HMP Pentonville is a category B Victorian prison which opened in 1842 and holds more than 1,200 adults.
According to a HM Inspectorate of Prisons report in February last year, the prison sees a rapid turnover, with more than 100 new prisoners a week and it was "performing poorly" as a result of staff shortages, overcrowding and prisoners' easy access to drugs.
Nick Hardwick, then chief inspector of prisons, also noted in his report: "Most prisoners felt unsafe; levels of violence were much higher than in similar prisons and had almost doubled since the last inspection."