Italian experts have raised doubts about DNA evidence that helped convict US student Amanda Knox for the murder of her British roommate Meredith Kercher in 2007, Ansa news agency reported yesterday.

In a leaked report, experts who have been carrying out tests as part of Ms Knox’s appeal against her conviction said they could not be certain that DNA traces found on the presumed murder weapon were in fact those of the victim.

They also said that DNA traces found on Ms Kercher’s bra clasp which the first trial linked to Ms Knox’s boyfriend at the time, Raffaele Sollecito, could have come from atmospheric contamination and pointed to mistakes in the tests.

“The international procedures for inspection, protocol and collection of evidence were not followed” for both the bra clasp and the knife, they said.

The original trial said the knife, which was found in Mr Sollecito’s house, had traces of Ms Knox’s DNA on the handle and of Ms Kercher’s DNA on the blade.

The findings are set to be formally debated at a hearing in the appeal trial of Ms Knox and Mr Sollecito, who were sentenced to 26 years and 25 years for the gruesome murder which shocked the Italian university town of Perugia.

A third person, Rudy Guede, has also been convicted for the murder as well as sexual assault of Ms Kercher. On Monday, he testified at the appeal trial of Ms Knox and Mr Sollecito claiming they were the only ones to blame for the killing.

Ms Kercher, an exchange student from Leeds University, was found in a pool of blood in the cottage she shared with Ms Knox. The case has been turned into a TV film in the United States starring Hayden Panettiere as Ms Knox.

Prosecutors have said they believe Ms Kercher was killed by Mr Guede, Mr Sollecito and Ms Knox as part of a drug-fuelled sex game that turned violent.

All three claim they are innocent of the murder, although Mr Guede has admitted he was in the house at the time of the killing.