Constitutional Court secretary Martin Guzman said the trial needs to go back to where it stood on April 19 to solve several appeal issues.
The ruling came 10 days after a three-judge panel convicted the 86-year-old Rios Montt of genocide and crimes against humanity for his role in massacres of Mayans during Guatemala's civil war. It found he knew about the slaughter of at least 1,771 Ixil Mayans in the western highlands and didn't stop it.
The tribunal sentenced him to 80 years in prison, drawing cheers from many Guatemalans. It was the first time a former Latin American leader was convicted of such crimes in his home country and the first official acknowledgment that genocide occurred during the bloody, 36-year civil war, something the current president, retired Gen. Otto Perez Molina, has denied.
Rios Montt's lawyers immediately filed an appeal and he spent only one day in prison before he was moved to a military hospital, where he remains.
A defense lawyer, Francisco Garcia Gudiel, told The Associated Press by telephone that he would seek the former dictator's freedom.
"There is no alternative," Garcia said. "The court has made a legal resolution after many flaws in the process.
The proceedings against Rios Montt were first suspended on April 19 after the tribunal hearing the oral part of his trial asked the Constitutional Court to decide if the proceeding should continue after another judge sought to have it annulled.
The trial had been nearing closing arguments when that judge, Carol Patricia Flores, intervened. Flores had been in charge of the first phase of the trial, in which evidence was gathered and determined to justify a trial, but she was removed from the case in February 2012 after defense lawyers charged her with bias.
Flores was reinstated to the case in early April by the Constitutional Court. She tried to halt the trial, but the tribunal hearing the case went ahead with testimony and convicted Rios Montt.
The tribunal issued its verdict after a nearly two-month trial in which dozens of victims testified about mass rapes and the killings of women and children and other atrocities committed by government troops.
Survivors and relatives of victims have sought for 30 years to bring punishment for Rios Montt. For international observers and Guatemalans on both sides of the war, the trial was seen as a turning point in a nation still wrestling with the trauma of a conflict that killed some 200,000 people.