Rome, March 25 - Italy's supreme Court of Cassation on Tuesday overturned the acquittal of Amanda Knox and Raffaele Sollecito for the 2007 murder of Meredith Kercher in Perugia and ordered a new appeal trial. The Cassation scrapped the Perugia appeals court's 2011 decision to quash the 26-year and 25-year sentences Knox and Sollecito were handed respectively at the original trial in 2009. The Italian court of last instance said the new appeal trial will take place in Florence. Knox, 25, said she was pained but she added that she was confident she would eventually be proclaimed innocent. "It was painful to receive the news that the Italian Supreme Court decided to send my case back for revision when the prosecution's theory of my involvement in Meredith's murder has been repeatedly revealed to be completely unfounded and unfair," Knox said in a statement issued via a family spokesman. "No matter what happens, my family and I will face this continuing legal battle as we always have, confident in the truth and with our heads held high in the face of wrongful accusations and unreasonable adversity," Knox said. The American, who is a student at the University of Washington, stayed up until 2 a.m. local time in her home city of Seattle to find out the decision. She could face a demand for extradition if the conviction is reinstated. Kercher's sister, on the other hand, wept tears of joy. ''I am happy and I want to understand,'' Stephanie Kercher told the lawyer representing the family, Francesco Maresca. Leeds University exchange student Kercher was found with her throat cut on November 2, 2007 in the house she shared with Knox in the central Italian town of Perugia. Rudy Guede, now 26, a native of the African nation of Ivory Coast, is in prison after being convicted separately from Knox and Sollecito and sentenced to 16 years in jail for the murder. In the final verdict against Guede, whose DNA was detected all over the murder house, he was found to have committed the crime "with others", during an alleged sex game that got out of hand. "This decision will help to get to the final, definitive truth about Meredith's murder," said Francesco Maresca, the lawyer representing the Kercher family. "There were other people with Guede. The judges will say who they are. This is a trial victory and a moral victory". Prosecutors said Monday that the judgement of Perugia's appeals court was "a rare concentration of violations of the law and of a lack of logic and I think (the acquittal decision) must be undone". In a detailed explanation of its decision, the Perugia court had said the evidence against the pair "does not allow us to come to the conclusion that guilt has been in any way proven". The Perugia court said that the evidence was largely circumstantial and prosecutors had been unable to prove motive. Knox's DNA was found on a knife prosecutors claim was the murder weapon, while Sollecito's was found on the victim's severed bra clasp. But the pair's defence have always claimed the DNA evidence was contaminated during the investigation. ''This trial has been an uphill battle from the start and we are climbing one step at a time,'' said high-profile Italian lawyer and former MP Giulia Bongiorno, a member of the team representing Sollecito, who turned 29 on Tuesday. ''Of course today, on Raffaele's birthday, we hoped to hear the final word, also because the appeal sentence was coherent and logical,'' she continued. ''However the decision by the Cassation does not represent a conviction. ''We will not start again from scratch. Only when we read the motivation for the sentence will it be possible to understand the grounds for the new appeals trial''. Courts often publish the explanations of their decisions months after they are announced. Knox's appeal against a three-year prison sentence for calumny was rejected by the Cassation, which means the conviction has become definitive. Knox has already served the sentence for having falsely accused a Perugia pub owner, Congo native Patrick Lumumba, in the early stages of the investigation.