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 American Knox in Perugia 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 US 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 Florence.
However it 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".
Return to Italy
"I've spoken to the family and Stephanie, her sister, is very happy, she's trying to understand what happens now," he said.
It is unclear if Knox, now 25, intends to return to Italy for the trial but in a statement issued through representatives, she said the decision was "painful".
The prosecution had repeatedly been revealed as "unfounded and unfair," she 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."
Battle
Tuesday's ruling by the Court of Cassation 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.
Kercher, from Coulsdon in Surrey, was on a year-long exchange programme in Perugia when she was murdered, bringing a flood of unwelcome attention to the medieval town in central Italy that her family said she loved.
Much of the attention of the case was focused on the carefree image of foreign students enjoying a year abroad in Italy as well as on lurid stories of sex and heavy partying.
Prosecutors had said that Kercher 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.
Guede, found guilty and sentenced to 16 years in a separate trial, is now the only person serving time for the murder, although prosecutors say he could not have killed Kercher by himself. "We are convinced there were more people in that room than Rudy Guede," Maresca said.
"We are asking the judges to tell us something on this point, as long as their decision is well-grounded and thorough, not like the appeals court which was absolutely superficial."
The defence argued that no clear motive or evidence linking the defendants to the crime had emerged, and said Knox was falsely implicated in the murder by prosecutors determined to convict her regardless of the evidence.
Giulia Bongiorno, a lawyer for 29-year-old Sollecito, said the decision was not a guilty verdict for her client but just meant the court wanted a more in-depth examination of some aspects of the case.
"Unfortunately we have to continue the battle," she told reporters. "This is a sentence that says, with regards to the acquittal, that something more is needed," she said. – Reuters