Ms. Knox and Mr. Sollecito were each sentenced to 25 years in prison for the crime, and Ms. Knox received an extra year for slander after she falsely accused another man of committing the murder.

But questions were raised during the appeal about the quality of the forensic evidence, as well as the reliability of some witnesses, and the prosecutors’ theory of the crime.

The convictions were overturned on appeal, and the two were released in October 2011, but last year prosecutors filed an appeal with the Court of Cassation. Ms. Knox’s lawyers appealed the charge of defamation, but the ruling on Tuesday upheld it.

In a statement issued by her media advisers within minutes of the announcement, Ms. Knox said it was “painful” to receive the court’s ruling “when the prosecution’s theory of my involvement in Meredith’s murder has been repeatedly revealed to be completely unfounded and unfair.”

Mr. Dalla Vedova said that the legal reasoning for ordering a new trial was expected from the Court of Cassation within 90 days, and that at that point, lawyers would learn “which points of the case will have to be re-examined” in the new appeals trial. “The trial starts from zero,” he said, “and after we see the decisions we will know whether certain witnesses have to be recalled, or evidence retested.”

If the new appeals court upholds the previous conviction and the Court of Cassation confirms it, Mr. Dalla Vedova said, Ms. Knox will have to serve out her sentence. The lawyer said Italian authorities would have to authorize an extradition request and the United States Justice Department would have to approve it.

There is no final ruling in any case until the Court of Cassation has signed off on it, Mr. Dalla Vedova said.