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 (eight inches) 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, in 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.