Family and friends are mourning the death of a promising art student from Hampstead who has died after inhaling laughing gas.

University College School (UCS) student Joseph Benett, known as Joe, suffered a cardiac arrest after inhaling nitrous oxide from a canister with his friends.

The 17-year-old, of West Heath Drive, lay in a coma for four weeks but died last Thursday with his family at his bedside.

Speaking to the Ham&High, his devastated 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. I found it hard to leave him. As a big sister I felt a responsibility to look after my little brother.”

Joseph, who would have turned 18 next month, was at his friend’s house in Hertfordshire when he collapsed after inhaling the popular party drug on August 31.

Despite the legal high being widely available at music festivals, bars and nightclubs, it is an offence to sell laughing gas for recreational use.

“I think having experienced how much this hurts, people should think about whether they are putting themselves in danger,” his sister said. You do need to think about what you are doing, not just for yourself but for the sake of the people who love you.”

The popular teenager was about to begin his final year at UCS independent school in Frognal, Hampstead, and was previously a pupil at Saint Christina’s primary school, in St John’s Wood, and St Anthony’s School, in Hampstead.

Staff and pupils at UCS have been shocked by the death of the sixth-former.

Headteacher Kenneth Durham said: “It is an absolute tragedy. His friends are terribly upset. He was very talented. We will remember Joe and support his family.”

Dr Alan McGlennan, a consultant anaesthetist at the Royal Free Hospital, said taking laughing gas recreationally was “in vogue” but it can starve the brain of oxygen.

An inquest into Joseph’s death was opened and adjourned at Hertfordshire Coroner’s Office on Monday.

He is survived by his photographer father, David, mother Rosann, and sister Camille. His funeral will take place at the Roman Catholic Church of St Mary’s, in Holly Place, Hampstead, next Thursday at 1pm.

Close family will then attend his cremation at Golders Green Crematorium, in Hoop Lane, Golders Green.

Donations should be made to St John’s Hospice, in St John’s Wood, which cared for Joseph, in lieu of flowers.