Luke Walmsley was stabbed in the chest

Dr David Rouse told Nottingham Crown Court the blow was more consistent with a stabbing action.

The 16-year-old defendant, who denies murder, claims Luke walked onto a blade intended only to scare him.

Luke, 14, died after being stabbed at Birkbeck School in North Somercotes, Lincolnshire, in November 2003.

The stab - him walking into the knife and it going into him - I didn't plan for it to happen The defendant

Dr Rouse was appearing for the defence but, under cross examination, when asked whether it was possible that no force was used by the person wielding the knife, replied: "No, not possible."

The defendant, who cannot be named, said the whole incident had been "totally unexpected" and denied making up a story to avoid admitting he had struck a blow to schoolboy's chest.

Yvonne Coen QC, prosecuting, suggested the idea that Luke had walked on to the blade was "complete nonsense".

"You never moved that blade from a foot in front of you?" she asked, "You never used any force whatsoever to put it in Luke's chest?"

"I can't say that because it was totally unexpected," he replied.

"The stab - him walking into the knife and it going into him - I didn't plan for it to happen."

Ms Coen then said: "There would have been no punching, no forceful blow, no swinging, no chucking a punch, no jabbing - none of those things?"

"No," he replied.

Ms Coen asked: "This account you are giving of keeping the knife completely still is something that you have made up because you know that to admit punching Luke in the chest with a blade is tantamount to murder itself, isn't it?"

The teenager replied: "No."