Art student Joseph Benett suffered a cardiac arrest after inhaling nitrous oxide in August. The teenager from Golders Green fell into a coma and then died last Thursday with his family by his side.

An interim Hertfordshire Coroner’s Court report gave the cause of death as hypoxic brain injury caused by inhalation of nitrous oxide, but a full inquest is due to be held in 2013.

His sister Camille, 27, said: “He just never woke up. He had such terrible brain damage and his fits were getting worse. When he stopped breathing, my dad and I were holding his hand”

Benett’s headteacher Kenneth Durham said: “They were playing with laughing gas and for some reason it had a negative reaction with serious consequences. He was a popular, happy boy with a good sense of humour and a very talented art student. It is particularly devastating for his family and close family.”

The sale of nitrous oxide for recreational use is prohibited by law.