ROME — Italy’s highest criminal court ordered a new trial for Amanda Knox and her former Italian boyfriend Tuesday, overturning their acquittals in the gruesome 2007 slaying of her British roommate.

The move extended a prolonged legal battle that has become a cause celebre in the United States and raised a host of questions about how the next phase of Italian justice would play out.

Italian law cannot compel Knox to return for the new trial, and one of her lawyers, Carlo Dalla Vedova of Rome, said she had no plans to do so.

“She thought that the nightmare was over,” Dalla Vedova said. “(But) she’s ready to fight.”

Knox, now 25 and a University of Washington student in Seattle, called the decision by the Rome-based Court of Cassation “painful,” but said she was confident that she would be exonerated.

The American left Italy a free woman after her 2011 acquittal, but only after serving nearly four years of a 26-year prison sentence from a lower court that convicted her of murdering Meredith Kercher. The 21-year-old British exchange student’s body was found in November 2007 in a pool of blood in the bedroom of a house the two shared in the Italian university town of Perugia. Her throat had been slit.

Raffaele Sollecito, Knox’s Italian boyfriend at the time, also was convicted, sentenced and later acquitted.

It could be months before a date is set for a fresh appeals-court trial for Knox and Sollecito in Florence.

Prosecutors alleged that Kercher was the victim of a drug-fueled sex game gone awry. Knox and Sollecito denied wrongdoing and said they weren’t even in the apartment that night, although they acknowledged they had smoked marijuana and their memories were clouded.