PHNOM PENH, Cambodia — Barely a month after an international tribunal in Cambodia convicted two senior Khmer Rouge leaders of genocide, its historic verdict was being thrown into question by an unexpected twist: an unlicensed lawyer.

The lawyer, Victor Koppe, has worked as defense counsel since 2007 for one of the convicted men, Nuon Chea. His client, now 92, was sentenced to life imprisonment by the international tribunal last month.

But Mr. Koppe was dismissed by the tribunal this past week after the bar association in Cambodia said that Mr. Koppe, a Dutch national, had not been a member of the bar in the Netherlands for nearly three years. Without his foreign registration, Mr. Koppe could not be licensed as a lawyer in Cambodia, the bar association said.

It remained unclear what effect, if any, this development would have on the genocide ruling, which is now in the appeals stage. The verdict came four decades after at least 1.7 million people — a fifth of Cambodia’s population — lost their lives under the Khmer Rouge.