The trial was put on hold as the panel considered a claim by defense attorneys that extensive publicity about the abduction has tainted the jury pool. Prosecutors have until 12:59 a.m. Friday to respond to the claim.
"I'm of course very unhappy about this," Kimball told the nine men and five women serving as jurors and alternates before he apologized and released them for the day. "I have to do what the 10th Circuit says. I don't know what they'll do when they consider it."
Kimball previously rejected defense requests to move the case from Utah.
Like federal prosecutors, the judge maintained an impartial jury could be found, based on the court's decision to call more than 500 prospective jurors and to prescreen their attitudes with a 42-page questionnaire.
News of the appeals court decision came shortly after defense attorney Parker Douglas began his opening remarks.
"It's horrible timing, no one should be stopped in the middle of opening arguments," he said.
It was unclear how quickly the court might rule or when the trial might resume.
Ed Smart, Elizabeth's father, said the family did not want to comment on the 10th Circuit decision.
Melodie Rydalch, a spokeswoman for the U.S. attorney, also declined comment.
In its petition filed Wednesday, the defense contended that three days of jury selection proved that pretrial publicity made finding an impartial panel a "practical impossibility."
The petition alleged the court was only able to impanel a jury by discounting the significance of the bias against Mitchell.
The filing included selected transcripts from jury questioning in which prospective jurors said they believed Elizabeth Smart had been abducted, held captive and raped by Mitchell.
Rebecca Woodridge, whose mother had been married to Mitchell, praised the appeals court's decision.
"People are biased in this state toward this case," she said. "They don't look at Brian as a person who deserves a fair trial. They look at him as a monster."
If Mitchell is convicted of the charges, he could spend the rest of his life in prison.
Elizabeth Smart's mother and her sister Mary Katherine, who was sleeping in the same bed with Elizabeth when she was taken at knifepoint, were at the courthouse and prepared to testify Thursday.
Elizabeth Smart is also expected to give lengthy, detailed testimony about her abduction, alleged sexual abuses during nine months of captivity, and threats she has said Mitchell made on her life and her family.
Smart was 14 when she was snatched from her bedroom in June 2002. She was found with Mitchell nine months later.