Luke Walmsley was stabbed in the chest in January 2003

Dr Lou-ay Al-Alousi told the jury a post-mortem examination had shown the stab wound to be 11cm deep.

Nottingham Crown Court heard the knife had touched the back of the heart.

A 16-year-old, who cannot be named for legal reasons, denies murdering Luke, 14, at the Birkbeck School, North Somercotes in Lincolnshire.

In my opinion the force is considerable because it didn't only cut the bone but also caused the hilt to imprint on the skin Dr Lou-ay Al-Alousi, Home Office pathologist

The pathologist said Luke was stabbed with such force that marks from the knife's hilt were left on his skin.

Luke, from Cleethorpes, died within hours of the knife attack outside a science class on 4 November 2003.

Dr Al-Alousi, told the jury at Nottingham Crown Court: "I feel very strongly that there are marks which represent hilt marks or screws.

"If this is true, then in my opinion the force is considerable because it didn't only cut the bone - the sternum or breastbone - but also caused the hilt to imprint on the skin."

He denied under cross-examination that they could have been caused by scissors used to cut off the boy's clothes.

Earlier the last of 19 child witnesses had given evidence by video link from Immingham.

A 16-year-old boy claimed the defendant and a friend had shown him a knife a month before Luke's death and boasted they were going to stab Luke with it.

The witness said he thought the boys were joking and did not seem serious.

The trial continues.