Because the phone is missing, the judge postponed a court hearing planned for Friday , June 21, to July 8. If the phone is not turned over by then, the judge said, the accuser will be asked to testify about its whereabouts. Mr. Garabedian, who declined to comment beyond his filing, wrote that the family planned to have a “digital forensic expert” search for backups of the phone’s contents.

R. Michael Cassidy, a law professor at Boston College, said that the phone’s disappearance could have significant consequences for the prosecution if the judge determines that officials associated with the case were at fault. If the judge were to determine that officials had deliberately disposed of the phone , he might dismiss the case — which Professor Cassidy said was unlikely — or he might inform the jury that it could hold the phone’s disappearance against the prosecution when considering the evidence.

A spokeswoman from the Cape and Islands district attorney’s office declined to comment, saying that the case “will be prosecuted in the courtroom, not the media.”

In 2016, the accuser was an 18-year-old busboy working at the Club Car, a restaurant and bar in Nantucket. More than a year later, he told investigators that he had been drinking with Mr. Spacey after a shift when Mr. Spacey put his left hand on his thigh and then unzipped his pants. The accuser said Mr. Spacey rubbed his penis for about three minutes.

The young man’s mother, Heather Unruh, a former television news anchor in Boston, said at a news conference that Mr. Spacey had sexually assaulted her son, and Mr. Spacey was eventually charged with indecent assault and battery.