Siegfried Blunk said he was stepping down because government objections to further prosecutions linked to the 1975-79 regime effectively made his position untenable.
He described “repeated statements which will be perceived as attempted interference by government officials” over his work on two possible new cases against five ex-Khmer Rouge members accused of crimes against humanity.
Blunk’s departure will come as an embarrassment to the tribunal, which was set up to bring some justice for the deaths of up to two million people but has been mired in controversy over its handling of the new cases.
The court has so far completed just one trial – jailing Kaing Guek Eav, a former Khmer Rouge prison chief, last year for 30 years for overseeing the deaths of 15,000 people.
A second trial involving the regime’s four most senior surviving leaders is under way and expected to start hearing testimony before the end of the year. The third and fourth cases are widely expected to be dismissed.
Blunk, the tribunal’s international co-investigating judge, had faced mounting criticism in recent months from observers and victims accusing him of bowing to political pressure to drop the two final cases.
Cambodian authorities have made no secret of their objection to pursuing new suspects, with premier Hun Sen – himself a former Khmer Rouge cadre – even saying prosecutions beyond the current second trial were “not allowed”.
A statement from Blunk, released through the court, said such government comments against the investigations had cast doubt over his reputation.
He said that while he would not have been influenced by political statements, his “ability to withstand such pressure by government officials and to perform his duties independently could always be called in doubt, and this would also call in doubt the integrity of the whole proceedings” for the new cases.
His resignation comes a week after Human Rights Watch called for both Blunk and his Cambodian counterpart You Bunleng to step down for failing “to conduct genuine, impartial, and effective investigations” into cases three and four.
The office of UN Secretary General Ban Ki-moon said Blunk would be replaced “as soon as possible” by a reserve judge.
The Khmer Rouge was responsible for one of the worst horrors of the 20th century, wiping out nearly a quarter of Cambodia’s population (some two million people) through starvation, overwork and execution in a bid to forge a communist utopia.