ROME - 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, according to media reports.

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 2011 after four years in prison. Both now face a retrial.

Prosecutor general Luigi Riello told the supreme court on Monday that the acquittal was "a rare mix of violation of the law and illogicalness and should be overturned".

"I believe all the premises are there to make sure the final curtain does not drop on this shocking and dire crime," he said, adding that the judge who acquitted Knox and Sollecito "lost his way".

Knox and Sollecito now face a re-trial in Florence although it is reported that Knox will likely be tried in absentia.

The Seattle student returned to her home town immediately after her acquittal and the United States does not normally extradite its citizens to face legal action.

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.

Her body was covered with knife wounds and bruises and investigators found traces of a sexual assault.

A third person, a local Ivory Coast-born drifter named Rudy Guede, who like the other two has always denied the murder, is the only person still in prison for the crime.

Kercher family believe more than one killer involved

The appeals judge found Knox and Sollecito not guilty in 2011, but said the killing still remained "unsolved" because investigators insist it must have been carried out by more than one person.

Kercher's family insists that 47 knife wounds on Meredith and the apparent use of two different knives in the attack meant that more than one killer was involved.

Prosecutors had alleged that Kercher was killed in a drug-fuelled sex attack involving Knox, Sollecito and Guede and had claimed that it was the American student who delivered the final blows while the other two held the victim down.

Knox was painted by her accusers as a seductive "she-devil" who had an unhealthy obsession with sex, while her defence insisted she was simply a naive girl-next-door whose nickname "Foxy Knoxy" referred to her childhood football skills.

In her first interrogation following the murder, Knox said she was in the house at the time and falsely identified the owner of a bar where she worked as the killer.

Knox's lawyer Carlo Dalla Vedova told the court on Monday that "this girl was stressed, confused and pressured" when she made the statement implicating the bar owner.

Knox later said that she was with Sollecito at his house all night and that her initial comments were misunderstood.

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 review cast serious doubt on the original analysis. American journalist and author Andrea Vogt spoke to Breakfast here on Newstalk this morning.