Duch is helping prosecutors describe the inner workings of the Khmer Rouge regime as they seek to tie three suspects to atrocity crimes.
His role in the rise of the Khmer Rouge has not so far been closely examined by the court, and he has never given testimony.
Nuon Chea is on trial for atrocity crimes alongside two other Khmer Rouge leaders, Khieu Samphan and Nuon Chea.
You Bunleng said in a written response Monday the Swiss judge had “committed professional mistakes” and had resigned as a result.
Sam Rainsy told VOA's Khmer service Thursday that officials in the government of Prime Minister Hun Sen "fear a serious investigation."
In a 14-page memo, Kasper-Ansermet said Cambodian staff followed the instructions of his Cambodian counterpart, You Bunleng, only.
Duch said Wednesday that Pol Pot and Nuon Chea “monopolized” decisions on “policy, commerce, the economy and the military.”
Kasper-Ansermet’s resignation throws further doubt of moving forward with a group of additional prosecutions known as cases 003 and 004.
During testimony Tuesday, Duch admitted he was “not clear” on the policymaking of the regime.
Duch tells the UN-backed court that the regime had a policy to “arrest and kill” perceived enemies and spies.
Duch is scheduled to testify at the court on Monday in the atrocity crimes trial of Nuon Chea, Khieu Samphan and Ieng Sary.
Cambodian prosecutors presented a list of experts to the Trial Chamber of the court who could affirm the authenticity of the documents.