The Italian appeals court that cleared Amanda Knox over the murder of British student Meredith Kercher has given the reasons for its ruling.

It said the evidence that had been used by a lower court to convict the American and her Italian boyfriend of murder did not hold up.

Those shortcomings included no murder weapon, faulty DNA, an inaccurate time for the killing, and insufficient proof that Knox and Raffaele Sollecito were even at the location where the crime occurred.

Ms Kercher was found dead in a pool of blood on her bedroom floor in Perugia, Italy, on November 2, 2007.

Knox and Sollecito, who had just begun dating at the time of the murder, were arrested several days later, then convicted in what prosecutors' portrayed as a drug-fuelled sexual assault. They were sentenced to 26 years and 25 years, respectively, in proceedings that made headlines around the world.

On Thursday, the court cited among the other failed elements of the prosecutors' case DNA evidence, which was undermined during a re-examination in the appeals trial, and the failure to conclusively identify the murder weapon.

The appellate court even contradicted the lower court's time of death, saying it happened at around 10.15pm and not after 11pm. The court said not only had the "building blocks" used to construct the case failed, but the material necessary to construct the case was missing.

The only elements of the prosecutors case that were proven, the appeals court said, were the charge of slander against Knox, who was convicted of falsely accusing a bar owner for Kercher's murder, and the fact that the Knox and Sollecito alibis did not match.

But that the alibis were out of synch "is very different" from the prosecutors' claim of false alibis, the court said.

The proven elements combined, the court said, are not enough to support convictions against Knox and Sollecito.