The decision by the Court of Cassation adds a further twist to a long-running case whose initial handling was sharply criticised by independent forensic experts.
Prosecutors accused Amanda Knox and her ex-boyfriend Raffaele Sollecito of assaulting and killing Kercher in 2007.
They were initially found guilty and sentenced to 26 and 25 years in prison respectively.
In 2011, their convictions were overturned and they were released after serving four years.
More top news
Consumers have been warned to immediately stop using Indesit tumble dryers after a machine reportedly caused a huge fire in a tower block.
Some 29 children have been killed in Aleppo in the week leading up to US-Russia talks which aid workers hope will lead to a vital ceasefire.
The 23-year-old was greeted by hundreds of fans as has returned to his gym in Essex for the first time since victory in Rio.