British student Meredith Kercher was the victim of a plan by her killers to "satisfy their sexual instincts" that then got out of control, a judge has ruled.

Judge Paolo Micheli made the ruling in a written explanation of his decision in October to convict one of the three suspects over the killing of Miss Kercher.

His conclusions were reported by the Italian news agency ANSA. In Italy, judges often issue explanations of their decisions months after they are handed down.

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Judge Micheli sentenced Ivory Coast national Rudy Hermann Guede to 30 years in prison after he underwent a fast-track trial last year at his request.

American Amanda Knox and her former Italian boyfriend Raffaele Sollecito are on trial on the same charges of murder and sexual violence over the death of Miss Kercher in the town of Perugia. All three have denied any wrongdoing.

Miss Kercher, 21, from Coulsdon, Surrey, was found stabbed to death on November 2, 2007, in the flat she shared with fellow exchange student Knox.

In the decision, Judge Micheli said Kercher was the victim of "an agreed-upon plan to satisfy sexual instincts" that got out of control, ANSA reported.

Judge Micheli said Guede participated "actively" in the assault but that there were others involved.

The ruling echoed the allegation by prosecutors that Miss Kercher was killed in what began as a sex game.

Prosecutors allege that Sollecito held her by the shoulders from behind while Knox touched her with the point of a knife.

They say Guede tried to sexually assault Kercher and then Knox fatally stabbed her in the throat.

At the start of Knox and Sollecito's trial earlier this month, Sollecito's lawyers argued that justice had already been served with Guede's conviction.

Lawyers for Knox and Sollecito say their clients were not in the apartment at the time of Kercher's murder.

The next hearing in the case is scheduled for 6 February.