Britain’s “youngest knife murderers” were on Friday sentenced to a minimum of eight-and-a-half years’ detention for killing a stranger in a machete attack when they were aged just 12.

The sentence was ordered as a nationwide ban on “zombie” style knives and machetes with blades of over 20 centimetres came into force this week to curb a wave of horrifying killings by young people.

The two boys, who are now aged 13 and cannot be identified legally because they are under 18, killed Shawn Seesahai, 19, in a park in Wolverhampton, central England, last November.

They were described in court as “the youngest knife murderers” and are also believed to be the youngest convicted of murder in Britain since 1993, when 11-year-old, Robert Thompson and Jon Venables were found guilty of killing a toddler, James Bulger.

Sentencing the pair, Judge Amanda Tipples told them: “What you did is horrific and shocking. When you killed Shawn he was 19, starting out in his adult life with everything to live for.

“His parents have lost their son. His sister has lost her brother.”

The court heard that the victim was not known to the pair but that they set upon him after he asked them to move from a bench.

Relatives of Seesahai, who was born in the British Overseas Territory of Anguilla in the Caribbean, described his murder as unexpected, senseless and committed “for no reason at all”.

Due to their age, the boys will not be jailed but held in secure accommodation. Once they are deemed fit for release they will be monitored for the rest of their lives.

There is mounting public concern about knife crime. In July, three girls aged six, seven and nine were killed in Southport, northwest England, in a mass stabbing that shocked the country.

Official figures lay bare the scale of the problem. In London alone, the number of knife or sharp instrument offences recorded by the police rose to more than 15,016 in 2023-24 from 12,786 in the previous year.

Prime Minister Keir Starmer this month launched an initiative to stop young people getting involved in knife crime, which he called a “national crisis”. He has enlisted the support of actor Idris Elba, who is pushing for government intervention to reverse drastic funding cuts to youth services as a way to end violence.

Even as the ban on “zombie” style weapons came into force on Monday, a 15-year-old boy died hours earlier after being stabbed in London.