A teenager chased down former Acland Burghley School pupil Lewis Blackman before stabbing him in the back, the Old Bailey heard today.

The defendant, who cannot be named due to his age, allegedly inflicted a “brutal” 12-inch wound that caused the victim to collapse moments later as he tried to escape an armed pack of young men,

The jury was told that the defendant, 17, had been the first to stab Mr Blackman, 19, and that the wound inflicted “on its own would have been fatal”.

A pathologist described the stab wound alleged to have been made by the defendant, saying: “It tracks through the skin, fat and muscle of the back before reaching the aorta. This resulted in a haemorrhage.”

The aorta is the largest artery in the human body.

Then, having watched CCTV that prosecutor Tom Little QC alleged shows the defendant stabbing Mr Blackman, who was known as “Dots” and “Dotty”, Dr Ashley Fegan-Earl told the jury: “In my view it would be consistent with the particulars of this injury – the one we have been describing.

“But it’s important to realise that when I make my examination I am dealing with a body on a table.”

Dr Fegan-Earl also explained as a result of damage to his aorta, the victim lost 1.7 litres of blood, confirming Mr Little’s assertion that “this injury on its own would have been capable of being fatal”.

Yet the prosecution argued this was the first of the 14 stab wounds the Kentish Town man suffered in the early hours of February 18 last year after he and a group of friends had gatecrashed a 16-year-old girl’s birthday party in Earl’s Court.

The pathologist confirmed only two wounds, both to Mr Blackman’s upper body, would have been fatal on their own.

The court was also shown CCTV of a figure said to be the defendant attacking the victim, before a number of young men also stabbed Mr Blackman as he lay dying. Further footage showed a group appearing to celebrate before fleeing the scene.

Det Con David Field talked the jury through the footage of the figures leaving the scene and explained his team had identified the defendant and five others from the footage and their clothes.

Images of the defendant attending the North-West London University Hospital with what appeared to be a heavily bandaged arm were also shown to the jury.

Opening the prosecution earlier this week, Mr Little told the court the killing was “truly shocking”, as he described a group of youths chasing Mr Blackman from the Earl’s Court Road party house, isolating him and then setting on him.

Late last year two other teenagers were convicted of Lewis Blackman’s murder and a third of his manslaughter.

The trial continues.