Amanda Knox's former lover threw doubt on her alibi for murder on Tuesday, saying a court ruling that found the pair guilty of killing a British student proved they were not together at the time of the crime.

Raffaele Sollecito unveiled his new defence strategy as the pair prepare to appeal their convictions for the bloody murder of Knox's former roommate Meredith Kercher, found with her throat slit in the Italian university town of Perugia in 2007.

The couple have been each other's alibis almost from the start, insisting they spent the night together at his house -- though they have admitted to smoking marijuana and being unable to recall everything they had done.

The 30-year-old Sollecito said he had "always believed, and still believe, that Amanda Marie Knox is innocent."

But he said the guilty verdict handed down by an appeal court in Florence this year was based on details which exonerate him -- while leaving doubts about her movements.

"The 25-year prison sentence given to me is based entirely on the statement written by Amanda Knox in pre-trial detention. She herself exonerates me entirely," he told a press conference in Rome.

In the text, the US citizen described "flashes of blurred images" in which she saw herself in the house while the murder was being committed, "with my hands over my ears because in my head I could hear Meredith screaming".

She makes no mention of Sollecito, which his defence invokes as proof he was not present at the scene.

Knox later recanted this version of events, saying she had written it under police pressure.

"Taking it as read that the judges are right (in basing their verdict on the statement) will you please explain what I have to do with it?" Sollecito said.

Shock about-turn

Sollecito's lawyer Giulia Bongiorno said their appeal would also revolve around a text message which Knox claims to have sent while with Sollecito at his house the night of the murder.

"This text message, according to the court's own ruling, was not sent from his house. Therefore the pair were not together," she said.

Sollecito is relying on the log from his home computer to prove he was home at the time of the killing.

While the Italian always claimed Knox "spent the night" with him, he never said they spent the evening together -- which is when the crime took place, Bongiorno said.

Asked where Knox may have been in those missing hours, Sollecito said he was concentrating only on defending himself and could not vouch for her whereabouts.

Kercher was found in a pool of blood in November 2007, half-naked and with her throat slashed. She had been stabbed 47 times.

Knox and Sollecito -- originally sentenced to 26 and 25 years in prison for the murder -- were acquitted on appeal in 2011 after four years behind bars, but then found guilty again in a shock about-turn by a Florence court in January.

The court said there was DNA proof that three people were at the murder scene: Knox, Sollecito and a third person, Ivory Coast-born drifter Rudy Guede, who is the only person still in prison for the crime.

Both Sollecito and Knox have argued the crime scene was contaminated and any DNA collected was therefore not viable.

The trial will next year go back to Italy's highest court, which could uphold the verdict or order a retrial.

If the verdict is upheld, officials are expected to begin a lengthy battle to extradite the 27-year-old Knox from the United States, where she returned on her release from prison in 2011.