Both men told the court hearing the pair’s appeal that Knox and Sollecito had nothing to do with the 21-year-old’s death in 2007.
Mario Alessi, serving a sentence for killing a child, told the hearing that a friend of co-defendant Rudy Guede had killed the Briton to silence her after she resisted a sexual assault.
The convict said Guede had spoken to him about the case during an exercise session at Viterbo prison where they shared a cell.
Guede allegedly told him Kercher was murdered by his friend, who was not officially identified.
“Rudy told me he tried to help the young woman [Meredith Kercher] after his friend wounded her with a small knife with an ivory-coloured grip,” Alessi told the court.
The friend reportedly then told Guede: “What are you doing? We have to kill her. If not we’ll end up in prison because of her.”
Alessi earlier made the same allegations in a deposition to police.
When questioned by Perugia prosecutors Guede, who was sentenced to 16 years in prison in a separate trial for his role in the murder, denied ever having made such statements.
Camorra boss Luciano Aviello, meanwhile, accused his own brother of the killing, telling the court his sibling killed Meredith Kercher in an attempt to steal a painting.
Knox, from Seattle, Washington, was sentenced in December 2009 to 26 years in prison for the murder of Kercher with whom she shared a house in Perugia.
Sollecito, Knox’s boyfriend at the time, was sentenced to 25 years for his part in the murder. Both are now appealing their convictions.
Prosecutors said they killed Kercher when a drug-fuelled sex game turned violent on the night of November 1, 2007.
She was discovered semi-nude in a pool of blood under a duvet with multiple stab wounds to the neck.
Ivorian Guede was convicted in 2008 and sentenced to 30 years in jail, but the appeal court later cut his term to 16 years.
At the court’s request, Guede is to be heard as a witness on June 27.
Knox and Sollecito were convicted largely because of traces of Knox’s DNA found on the knife that killed Kercher and Sollecito’s DNA on a bra clasp.
Knox’s parents were also ordered to stand trial for alleging that their daughter was beaten by Italian police in an interview they gave to Britain’s Sunday Times in 2008.