Italy's highest court of appeal overturned the acquittal of US student Amanda Knox on Tuesday and ordered a retrial over the murder of her British housemate in what prosecutors said was a drug-fuelled sex attack. Knox and her Italian former boyfriend Raffaele Sollecito -- originally sentenced to 26 and 25 years in prison for killing and sexually assaulting Meredith Kercher in 2007 -- were acquitted on appeal in 2011after four years in prison. Both now face a retrial in a Florence court after judges upheld a 2012 prosecution appeal against their acquittals. The Seattle student is "disappointed but has not lost heart. She knows she is innocent," Knox's lawyer Carlo Dalla Vedova told journalists outside the court house in Rome. "She is upset... (but) ready to continue, and willing to fight," he said. The 25-year-old -- who did not sleep a wink waiting for the court's decision, according to Dalla Vedova -- has not yet decided "whether she'll want to be present for the trial or not." Knox returned home to the US immediately after her release and, should she not return to Italy, will likely be tried in absentia as the United States does not normally extradite its citizens to face legal action. "It's not been easy from the start," said Sollecito's lawyer, Giulia Bongiorno. "We have had to climb a mountain, but we draw great strength both from being innocent and from the fact the court's ruling today is not a guilty verdict," she said. "The retrial means the court has decided some details need to be reviewed. The battle continues," she added. Sollecito, who turned 29 on Tuesday, told another of his lawyers Luca Maori: "I am disappointed. But I am innocent and can go on with my head held high." Prosecutors addressing the court on Monday had said they were convinced the former lovers were guilty of murdering Kercher. Calling for the judges to "make sure the final curtain does not drop on this shocking and dire crime," they said the acquittal, which was based mainly on the admissibility of DNA evidence in the case, contained "omissions and many mistakes". Prosecutor general Luigi Riello had described it as "a rare mix of violation of the law and illogicality and should be overturned," and accused the appeal judge of having "lost his way". Kercher, 21, was found half-naked with her throat slashed in a pool of blood in her bedroom in the house that she shared with Knox in November 2007. A third person, Ivory Coast-born drifter Rudy Guede, who like the other two accused has always denied the murder, is the only person still in prison for the crime. Kercher family lawyer Francesco Maresca punched the air in victory as the court's decision was read out, according to journalists present. "This decision serves to review the definitive and final truth of Meredith's murder. Guede was not alone, the judges will tell us who was there with him," he said. Kercher's older sister Stephanie Kercher said her family welcomed the court ruling. She told Sky News there were "still questions that are unanswered and we are all looking to find out the truth". She added: "We welcome the decision that a retrial has been ordered and are pleased it is a step forward to finding an answer to some of those questions." They -- and investigators -- insist that 47 knife wounds on Meredith and the apparent use of two different knives in the attack meant that more than one killer had been involved. Prosecutors had alleged that Kercher was killed in a drug-fuelled sex attack involving Knox, Sollecito and Guede. They had said that it was the American student who delivered the final blows while the other two held the victim down. The key to the appeal was an independent analysis of two pieces of evidence that had helped convict Knox and Sollecito -- a kitchen knife and Kercher's bra clasp. The appeals judge quashed the convictions of Knox and Sollecito in 2011 largely over the admissibility of DNA evidence. The review cast serious doubt on the original analysis, with experts and video evidence pointing to sloppy practice among the police at the crime scene and possible contamination of the evidence. Knox has been repeatedly painted by her accusers as a seductive "she-devil" who had an unhealthy obsession with sex, while her defence has insisted she is simply a naive girl-next-door, a yoga lover whose nickname "Foxy Knoxy" referred to her childhood football skills. In her first interrogation following the murder, Knox said that she was in the house at the time, and falsely identified the owner of a bar where she worked as a waitress as the killer. She later said she was with Sollecito at his house all night and blamed her initial comments on exhaustion and police coercion.