Amanda Knox delivered the fatal blow in the murder of Coulsdon student Meredith Kercher, according to judges in Italy.
The judges who reinstated the murder convictions of Knox and her Italian ex-boyfriend Raffaele Sollecito said they believed Miss Kercher was killed during a violent argument over money.
Miss Kercher, 21, was found dead in the flat in Perugia she shared with American Knox in 2007.
Knox and Sollecito were found guilty of her killing in 2009 before being acquitted on appeal in 2011.
But an appeals court in Florence overturned the acquittals in January this year.
Outlining their reasoning this week, judges explained Miss Kercher's wounds had been inflicted by multiple attackers.
Both Knox and Sollecito are believed to have wielded knives while Rudy Guede - a man from Ivory Coast jailed for the murder in 2008 - held down Miss Kercher down and sexually assaulted her.
But presiding judge Alessandro Nencini said Knox had "delivered the only mortal blow" with a kitchen knife.
The judges supported prosecutors' theory that Miss Kercher had died during a violent argument, having quarrelled with Knox about money earlier in the night she died.
In a statement released yesterday, Knox said the court's verdict was "not supported by any credible evidence or logic".
Both she and Sollecito are expected to appeal against the decision to reinstate their convictions and neither have yet returned to jail.
The trial will go to the highest court in Italy, the Court of Cassation, which can uphold or overturn the verdict.
Guede is currently serving 16-year prison sentence.
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