Italy's top court on Friday annulled the conviction of American Amanda Knox for the 2007 murder of British student Meredith Kercher and, in a surprise verdict, acquitted her of the charge.

The brutal killing and tangle of trials that followed gripped attention on both sides of the Atlantic, inspiring books and films. Kercher's family said Meredith, who died aged 21, risked being forgotten.

The Court of Cassation threw out the second guilty verdict to have been passed on Knox and her Italian former boyfriend Raffaele Sollecito for the lethal stabbing, saying there was insufficient evidence to convict either of them.

It had been widely expected that, even if the court overturned the previous convictions, it would order a retrial. Instead, both Knox and Sollecito are now definitively cleared.

"I cannot tell you how I feel in this moment," said her lawyer Luciano Ghirga outside the Rome courtroom after the verdict before he called Knox to tell her the news.

"I personally feel overjoyed that the truth has won out, that she is innocent," said David Marriott, a spokesman for Knox in her home town of Seattle.

Prosecutors had asked for jail sentences of 28 years and three months for Knox, and 24 years and nine months for Sollecito.

Knox, 27, and Sollecito, 31 have both already served four years in jail each after an original conviction in 2009.