It was initially suspected the cause of his death was sudden adult death syndrome, but eight months later Alex’s family learned he had taken a substance contaminated with a nitazene.

Phone records suggested he had tried to buy tablets usually sold as Xanax, which are only available with a private prescription in the UK.

Ms Jacques, who lives in north Wales, believes Alex was doing so because he often struggled with sleeping while taking medication for his attention deficit hyperactivity disorder.

The traces of nitazenes were only detected after she queried with police why earlier tests had not looked for them.

Ms Jacques said she remained in “disbelief” at the lack of testing, adding: “If I hadn’t pushed for better answers in the middle of massive grief, then to this day I would have no idea how he actually died.

“Unless we’re testing for them, how is anyone going to be aware and informed [of the dangers]?”

A Scotland Yard spokesperson said there had been “delays beyond the control of the Met” relating to the need for, and timing of, specialist testing in this case.

The North London Coroner Service said it remained in contact with the family regarding their concerns.