The Prison Governors Association (PGA) has renewed calls for a public inquiry into the state of jails in England and Wales following the death of an inmate.
Emergency services were called to Pentonville Prison in north London at 3.30pm on Tuesday, where they found three male prisoners suffering from stab wounds.
One of the prisoners, a man aged in his 20s, was pronounced dead at the scene at 4.25pm.
The two other men, aged 21 and 30, were said to be in a critical condition at an east London hospital.
Two prisoners, aged 26 and 34, have been arrested and taken to a local police station for questioning.
A spokesman for The Prison Service confirmed the incident was being investigated but said it would be "inappropriate" to comment further "at this stage".
The PGA said government cuts to staff and resources meant the "tragedy" was "no massive surprise".
The most recent statistics published by the Ministry of Justice (MoJ) show there were 100 apparently self-inflicted jail deaths in the year to March - the highest for more than a decade.
More than 20,000 assaults - 2,813 deemed "serious" - were recorded in the 12 months to December, a rise of 27% year-on-year.
In addition, there were nearly 5,000 attacks on staff - a jump of more than a third compared to 2014.
John Attard, PGA national policy officer, said: "It is no secret that we have had concerns about cuts and resources over the last four years.
"Sadly, it comes as no massive surprise to anybody close to this that we have had a tragedy such as this."
Mr Attard said "years of experience" had been lost, which was "showing".
"It's why we need an inquiry into this," he said.
In July, prison officers across England and Wales staged unofficial walkouts amid claims the system was in "perpetual crisis".
It came a month after the MoJ announced nearly £13m in funds to deal with prison safety issues.
In a statement the Prison Officers Association said it "once again" has serious concerns following the Pentonville death.
It has called on the MoJ to "fully investigate this matter and the underlying problems within the prison estate".
A Ministry of Justice spokesperson said the department was "fully committed to addressing the significant increase in violence, self-harm and self-inflicted deaths in our prisons".
They continued: "Earlier this month the Justice Secretary announced an immediate investment of an additional £14m in 10 of our most challenging prisons, increasing staffing levels by over 400 prison officers.
"In the coming weeks she will be publishing a White Paper setting out plans across the estate for prison safety and reform to 2020 and beyond."
Pentonville, a category B men's prison, was opened in 1842 and holds more than 1,200 adults.
In a report last February it was said to be "performing poorly" as a result of staff shortages, overcrowding and inmates' easy access to drugs.
Nick Hardwick, then chief inspector of prisons, also noted: "Most prisoners felt unsafe; levels of violence were much higher than in similar prisons and had almost doubled since the last inspection."