Police have warned how “easy” it is for teenagers to buy weapons and urged parents to check purchases after a youth killed a 16-year-old boy with a “ninja-style” sword.

The 17-year-old boy, who cannot be named for legal reasons, was found guilty of murder following a trial at the Old Bailey on Friday, after slashing Rahaan Ahmed Amin in the chest on July 9 in retribution for an earlier stabbing, jurors heard.

It emerged that an identical red knife to the weapon used in the killing had been ordered on the internet through online shop DNA Leisure in the weeks before the attack, on June 12, according to Scotland Yard.

Three orders had been made with a passport belonging to the father of one of the defendant’s friends who told police he had no knowledge of the purchase, the court heard.

DNA Leisure says on its website that age-restricted products are only dispatched after verification checks.

The last order was made with the defendant’s home as the delivery address.

Metropolitan Police Detective Chief Inspector Kelly Allen said Rahaan was killed in “cold blood”.

She went on: “This case demonstrates how easy it is for young people to purchase deadly knives online. I would urge all parents to be aware of their child’s online activity and what purchases they are making.

“It is also important for parents to keep their ID documents secure to ensure they are not misused by their children.”

Prosecutor Kate Lumsdon KC had told jurors how the defendant had cycled into West Ham Park in Newham, east London, with his face covered.

He had produced a long red knife and attacked Rahaan before making off.

Rahaan and his attacker were said to be standing “toe-to-toe” before the knife was used, while another witness said he saw a boy coming towards him with his hands over his chest shouting “save me, save me please”.

Paramedics were called to the scene and Rahaan was taken to hospital where he died the next day.

Police recovered the murder weapon bearing the defendant’s fingerprints and the victim’s blood on the blade hanging in a tree in the park.

They also uncovered a Snapchat photograph of nine knives lined up on the defendant’s bed, one of which looked like the red sword used in the killing.

Jurors were told that the attack came amid tensions between the defendant and victim’s groups of friends.

When one of the defendant’s friends was stabbed, Rahaan had been arrested on suspicion of involvement.

Snapchat messages suggested the defendant and his friends wanted to harm Rahaan in retaliation, the court was told.

Giving evidence in his trial, the defendant claimed he used the knife in self-defence.

Sentencing was adjourned until September 6 for reports to be prepared.

DNA Leisure has been approached for comment.