A 14-year-old boy shouted “I’m gonna wet you” before stabbing a teenager deep in the chest, a murder trial has been told.

Tomasz Oleszak, also 14, collapsed soon after the fatal wound was inflicted at a nature park in Gateshead, Newcastle Crown Court heard.

The defendant, who is now 15 and cannot be identified for legal reasons, denies murder and attempting to cause grievous bodily harm to a different boy whose coat was slashed when trouble flared last October.

He has previously admitted carrying a blade.

Mark McKone KC, prosecuting, said the incident began when the defendant and his girlfriend came across a larger group in Whitehills Nature Park.

The boy gave someone a “dirty look” and this provoked someone in the larger group of around 10 young people to say they were going to hit the defendant, the court heard.

As Tomasz approached, the defendant took out a knife and stabbed him above the heart, entering his aorta with a wound that was 3.2in (8cm) deep and 1.8in (4.5cm) wide.

Mr McKone said one of the group of youths later told police: “I heard the suspect shout ‘I’m gonna wet you’ – it’s slang for ‘I’m gonna stab you’.”

The prosecutor added: “The prosecution say that the phrase ‘I’m gonna wet you’ is the phrase of someone wanting to stab someone and not the words of someone reluctantly acting in self-defence.”

Another juvenile witness, who cannot be identified, told police: “I didn’t see it because I was at the back, but I heard someone shout ‘Your boy’s been wetted’ – it means ‘Your boy has been stabbed’.”

Mr McKone said the defendant later told his girlfriend: “If anyone messages you, say nothing, don’t say what’s apparently happened.”

The prosecution said the defendant phoned a friend urgently to get him to meet up after the stabbing.

The friend told police later: “He was bouncing all over, he seemed happy that he had done it, he looked like himself, it didn’t faze him at all.”

Mr McKone said the boy will claim self-defence.

He said: “The prosecution accept that some of the group moved towards Tomasz but they do not accept that the defendant was punched and kicked.”

He added: “The prosecution say that this was a deliberate stabbing and the defendant knew very well that he had stabbed someone, hence the evidence of (the friend).”

Mr Justice Spencer told the jury he and the barristers were not wearing their normal, formal dress and wigs due to the age of the defendant.

The trial is expected to last 10 days.