Italy's top court has overturned the 2011 acquittal of American student Amanda Knox and her former boyfriend Raffaele Sollecito for the murder of Briton Meredith Kercher, and ordered a retrial.

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 Ms Knox and Mr Sollecito of assaulting and killing Ms Kercher in 2007.

They were initially found guilty and sentenced to 26 and 25 years in prison respectively after a trial that grabbed headlines all over the world.

In 2011, their convictions were overturned and they were released after serving four years in prison.

Rudy Guede from the Ivory Coast was convicted of the killing in a separate case and is serving a 16-year sentence.

Ms Knox has said it is "painful" to have her acquittal overturned, but said she is confident in the truth.

She said any remaining questions about her case must now be examined by objective and capable prosecutors.

Ms Knox said the Perugia prosecutors' theory of her involvement in the murder is "completely unfounded and unfair".

She said: "No matter what happens, my family and I will face this continuing legal battle as we always have, confident in the truth and with our heads held high in the face of wrongful accusations and unreasonable adversity."