Duch ran a Khmer Rouge prison where almost all inmates were killed
Judges at the Khmer Rouge tribunal in Cambodia have ruled that a former prison chief, known as Comrade Duch, should not face additional charges.
He already faces charges of crimes against humanity, but prosecutors wanted to add conspiracy charges too.
They argued this would make it easier to prosecute other Khmer Rouge leaders.
The ruling means the final obstacle to starting the trials has been lifted, although Duch is unlikely to appear in court again until at least next March.
Duch's trial would have been well under way by now but for an appeal by the prosecutors.
They said that even though the former prison chief faced charges of war crimes and crimes against humanity, the scope of the indictment was not wide enough.
Specifically, they wanted him to stand accused of being part of a joint criminal enterprise with other Khmer Rouge leaders - a conspiracy to murder, starve and torture Cambodians in the late 1970s.
Judges rejected this, but they did widen the scope of the charges to include torture and pre-meditated murder.
The prosecutors may be disappointed, but for survivors of the Khmer Rouge regime, the more important factor is that a start to the first trial is now in sight, says the BBC correspondent in Phnom Penh, Guy Delauney.
The tribunal will name the date next week, and it is likely to be in the first quarter of next year.
As many as two million people are thought to have died under the Khmer Rouge. People were forced from the cities to work in the fields. Many died from starvation and overwork, while others were executed.
Our correspondent says that Duch may help to resolve the puzzle which has nagged away at Cambodians for so many years - why did Khmer kill Khmer, so often and so brutally?
The former prison chief has been cooperating with investigating judges - and the signs are that he will provide information about how the Khmer Rouge leadership made the decisions which led to so many deaths.