A friend of 17-year-old Vaso Kakko has described how the boy’s killer launched an attack from behind “suddenly and without warning”, fatally stabbing him in his neck and back.

Jurors at the Old Bailey heard how Uche Chukwu, 20 – also known by his street name Realz – stabbed Vaso, from Canonbury, just before 8.45pm on November 23, in a small community garden off Yoke Close on the Shearling Way Estate, Holloway.

The 6cm-deep neck wound severed his jugular and ceratoid veins and his voice box, and would have “bled rapidly and quickly become life-threatening” according to the pathologist. Another 9cm wound to Vaso’s lower back contributed to his collapse.

Paramedics and the Air Ambulance tried to save the former Mary Magdelene student’s life at the scene but he was dead within 35 minutes.

Opening the case on Tuesday, prosecution barrister Timothy Cray said: “The presence of the two long stab wounds tell you what this defendant was trying to do – in those moments when he got the knife out and used it twice he was trying to kill or at least cause really serious harm.”

Chukwu – who fled the scene and was arrested seven days later in Bognor Regis, Sussex – admits possession of the knife and stabbing Vaso, but denies murder, claiming he acted in self defence.

The Old Bailey was told how the defendant, victim and the two witnesses to the attack – Ryan Kelly and another youth called Gio – were a “group of friends and acquaintances who knew each other who would meet in this Caledonian Road area”, and had been smoking cannabis together that night.

Mr Kelly, who admitted he was “very nervous” as he gave evidence behind a screen, described how in the moments before the attack there had been “general chit-chat”, and Vaso had been showing a video of “a girl being flirtatious”.

He alleged Vaso had been facing away from Chukwu when the 20-year-old attacked him with what looked like a kitchen knife with a seven-inch blade, as Gio screamed.

He said he had not believed there would be any trouble “of that nature”.

Mr Cray said: “They [the two witnesses] say in that garden area the defendant approached the victim and attacked him suddenly and without warning. It was over within seconds. He then fled the scene.

“The point we would underline is they don’t seem to be describing a fight or even an argument – rather it sounds much more like a surprise attack where the victim is taken by surprise and off-guard with sudden and overwhelming force, before the defendant runs off.”

In his opening speech Mr Cray described how Chukwu claims the incident was “part of a feud between him and the victim”.

Mr Cray said: “Close to the time he alleges Vaso threatened the defendant with a taser – an electronic stun gun – and later that same evening Vaso stabbed the defendant in the leg.

“Certainly the defendant did go to the Whittington Hospital on February 17 with a wound on his leg. He didn’t complain to the police and told hospital he had fallen over on some glass when he was drunk.”

He continued: “Did he take his time and opportunity to take this overwhelming revenge?

“We say that whatever the cause, nothing justifies going up to someone and sticking a knife into their neck – that’s not how we settle our disputes in this country.”

The murder weapon was never been recovered.

The Old Bailey was shown pictures of Vaso’s body inside the forensic’s tent at the scene of the crime. His silhouette was blacked out so as not to cause distress but a knife could be seen sticking out from his jacket.

Mr Cray said: “It looks certainly as if Vaso was carrying a knife but it looks as though his knife was inside his jacket, still in its sheath.”

The trial continues.