Thomas Hardiman and Xander Howarth, now both 18, were sentenced to a minimum of 19 years each in prison for the murder of Adam Abdul-Basit earlier this year.

Howarth and Mr Abdul-Basit had fallen out over drug dealing, Sheffield Crown Court heard on Friday.

The defendants, who "harboured an intention to attack Abdul-Basit should the opportunity arise", set upon him with large knives, after chasing him into a residential garden in the Richmond area of the city in May.

Mr Abdul-Basit was left with unsurvivable injuries, including a 15cm (6-inch)-long wound to his chest.

Sentencing the pair, the Recorder of Sheffield Judge Jeremy Richardson KC said: "Those who carry knives in public are a scourge on our society and must be punished severely, in my judgment.

"The situation is made worse when, as here, the violence was closely allied with a fallout over drugs and drug dealing."

The judge acknowledged that both teenagers had terrible upbringings, telling Howarth: "It is small wonder you descended into the swamp of drug usage and drug dealing."

But Judge Richardson said: "You both had knives with you and you had them for violence should the need arise and should you feel the necessity to use those weapons.

"Each of you had an easy way with knives and you were both prepared to use them.

"These knives were large and could be used to inflict murderous and grievous injuries.

"Both of you used those knives with ferocious vigour upon the deceased."

The judge said: "Three young lives wrecked by knife crime related to drug dealing."

In a statement read to the court, Mr Abdul-Basit's mother Ferzana Afsar said her son was "my precious diamond".

Ms Afsar said: "Adam was the light of my life, he was my world, my light, my everything."

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After the defendants were found guilty of murder by a jury earlier this month, Detective Chief Inspector Phil Etheridge, of South Yorkshire Police, said Mr Abdul-Basit "died all alone on a stranger's doorstep while his attackers showed complete cowardice in fleeing the scene and trying to evade justice".

Hardiman, of Edenhall Road, Sheffield, and Howarth, of Richmond Park View, Sheffield, were both 17 at the time of the killing.

They were caught on CCTV wielding knives as they chased their victim into the front garden of a property in Smelter Wood Road, officers said.

Howarth, who was wearing a balaclava and stab vest, walked towards Mr Abdul-Basit with a machete raised over his head before swiping in his direction, they added.