* New trial to be held at Florence court
* Amanda Knox says decision is "painful"
* Kercher family lawyer hails "important day" for Italy justice
Italy's top court on Tuesday ordered a retrial of American Amanda Knox and former boyfriend Raffaele Sollecito for the murder of British student Meredith Kercher, re-opening a case that prompted harsh criticism of the Italian justice system.
Kercher's half-naked body, with more than 40 wounds and a deep gash in the throat, was found in the apartment she shared with Knox in Perugia, where both were studying during a year abroad in 2007.
Prosecutors accused Knox and Italian Sollecito of killing the 21-year-old Leeds University student during a drug-fuelled sexual assault that got out of hand.
The two, who always protested their innocence, were initially found guilty in 2009 and sentenced to 26 and 25 years in prison respectively after a trial that grabbed headlines around the world.
In 2011, their convictions were quashed after forensic experts challenged evidence in the original trial, prompting accusations of a botched police investigation and leaving many aspects of the killing unexplained.
They were released after four years in prison and Knox returned to her family home near Seattle immediately afterwards.
On Tuesday, the Court of Cassation overturned the acquittal and accepted a request for a retrial from prosecutors and Kercher family lawyers who had criticised the earlier ruling as "contradictory and illogical".
Unlike law in the United States and some other countries, the Italian system does not contain so-called "double jeopardy" provisions that prevent a defendant being tried twice for the same offence.
The court has not yet provided a full reasoning of its decision and a date has not yet been set for the new trial, which will be held in an appeals court in Florence, rather than Perugia, where the original trials were conducted.
The decision was immediately welcomed by the Kercher family lawyer Francesco Maresca who said it would provide an opportunity to find out what happened to Meredith.
"This is an important day for the Italian justice system," he said outside the court, criticising the earlier judgment acquitting Knox and Sollecito as "extremely superficial".
"I've spoken to the family and Stephanie, her sister, is very happy, she's trying to understand what happens now."
Knox released a statement through her spokesman David Marriott describing the court's decision as "painful" and said the prosecution's theory had repeatedly been revealed as "unfounded and unfair,".
She has not yet discussed whether she will return to Italy for the trial, Marriott said.
Knox, dubbed "Foxy Knoxy" in many early media reports, was initially portrayed as a sex-obsessed "she devil" by prosecutors but a lobbying campaign by her family helped change perceptions and she is due to publish a book of memoirs in April.
"She was very sad, she thought that this nightmare was over," Carlo della Vedova, one of her legal team told reporters after speaking to Knox. "At the same time she is ready, we went through all this before, we are strong enough and strong enough to fight again."
Tuesday's ruling examined whether there were procedural irregularities which gave grounds for a retrial, rather than assessing the details of the case, which remain obscure in many particulars.
Maurizio Bellacosa, a criminal law professor at Rome's LUISS University, said he expected the new trial would begin in less than a year.
If Knox is convicted of murder in the new trial, her lawyers will be able to appeal again, said criminal law expert Graziano Cecchetti from Italian law firm Giambrone Law.
Both experts said for now Knox was free to decide herself whether to return to Italy or not but the Italian government could request extradition if she is found guilty of the murder and her conviction is backed by the Court of Cassation.
Much of the attention of the case was focused on the carefree image of foreign students enjoying a year abroad in the medieval town in central Italy as well as on lurid stories of sex and heavy partying.
Prosecutors had said that Kercher was held down and stabbed after she resisted attempts by Knox, Sollecito and a third man, Ivorian Rudy Guede, to involve her in an orgy in the apartment the two women shared in the town.
However their case was weakened by forensic experts who undermined the credibility of DNA evidence provided by police and made strong criticisms of their first response procedures at the scene of the killing.