Nuon Chea, a key leader in Cambodia's Khmer Rouge who was convicted of genocide last year, has died at the age of 93.
Known as Brother Number Two, Nuon Chea was second only to the regime's leader Pol Pot during Khmer Rouge rule over Cambodia from 1975 to 1979.
Up to two million people are thought to have died during the Khmer Rouge's four years in power.
In 2018, a UN-backed court sentenced Nuon Chea to life in prison.
A spokesman for the Extraordinary Chambers in the Courts of Cambodia (ECCC) confirmed that the 93-year-old had died in hospital. The cause of death was not announced.
After seizing control of Cambodia in 1975, the Maoist Khmer Rouge attempted to force the country back to the Middle Ages.
Millions died from starvation, disease, overwork and execution - almost a quarter of the population. Nuon Chea was one of the ideological architects of this Year Zero policy.
After Vietnam invaded and deposed the regime in 1979, Nuon Chea fled to the hills with supporters until he was pardoned by the state in 1998 under the terms of a peace deal.
But under international pressure, Cambodia arrested the former leaders in 2007. Nuon Chea and former head of state Kieu Samphan were later convicted on charges of genocide and crimes against humanity.
Another of the regime's leaders, Kaing Guek Eav - known as Comrade Duch - was also given a life sentence by the UN-backed court in 2012.
A remorseless perpetrator of genocide
By George Wright, BBC News
While for most, Pol Pot is the name synonymous with the deaths of around two million Cambodians in the 1970s, Nuon Chea was just as important a cog in the Khmer Rouge's genocidal machine.
Defiant until the end, the regime's Brother Number Two refused to ever accept responsibility or show genuine remorse for the crimes committed on his watch.
"If we did not kill the internal traitors in our party, if we did not smash the enemy, there would be no Cambodia today," he once told a journalist.
Indicative of the paranoid nature of the regime, Nuon Chea blamed the majority of the killings on rebellious factions within the ranks of the Khmer Rouge who defied orders, although little credible evidence of this has ever been presented.
Nuon Chea saw himself as a patriot and a man of conviction. But history will remember him as a ruthless leader who oversaw the deaths of around a quarter of his own people in some of the worst crimes of the 20th Century.
George spent three years reporting on the Khmer Rouge tribunal