An Italian coroner has declared the knife lawyers claim was used to kill Meredith Kercher does not match her wounds.

Miss Kercher’s housemate American Amanda Knox and her ex-boyfriend Raffaele Sollecito deny murdering the Coulsdon student in a sex game that went wrong in November 2007.

A third person, Rudy Guede, an Ivory Coast drifter was convicted of the murder and sentenced to 30 years in prison in a fast track trial last October.

One of the key pieces of evidence in the trial, which is taking place in Perugia, Italy where Miss Kercher was studying, is a 6 1/2-inch kitchen knife found at Mr Sollecito's house.

The prosecution said it has Miss Knox’s DNA on the handle and Miss Kercher’s DNA on the blade.

However, coroner Francesco Introna told the court last week one of the cuts in Miss Kercher’s neck was made by a blade about three inches long.

Amanda Knox’s lawyers have said it’s natural the handle of the knife would have her DNA on it because she used it to cook dinner at Mr Sollecito’s house.

They claim Miss Kercher’s DNA on the blade is as a result of a bungled police investigation in which forensic evidence was contaminated.

Dr Introna's evidence contradicts the opinion of a forensic expert, Gianaristide Norelli, who told the court earlier this month the wounds to Miss Kercher's neck were compatible with the kitchen knife.

Dr Introna also said it was impossible for Miss Kercher to have been killed by three people because her room was too small for four people to interact the way the prosecution describe.

Prosecutors allege she was killed during what began as a sex game, with Mr Sollecito holding her by the shoulders from behind while Miss Knox touched her with the point of a knife.

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

Amanda Knox’s mother testified at the trial last week.

Edda Mellas said her daughter got on very well with the victim and she told her mother about “the fun things she and Meredith did".

She also told the jury her daughter called her three times on the day of the murder, the first to tell her there was an intruder in the house and the second and third after Miss Kercher’s body had been discovered.

She said she was very upset.

Mr Sollecito’s father also testified telling the court his son would not hurt a fly and he described his romance with Miss Knox as a “beautiful love story”.

The trial continues.