A seven-year-old boy died at the private Portland Hospital in London after vital equipment to monitor his breathing was switched off, a report says.
James Dwerryhouse suffered brain damage as a result of cardiac arrest after the equipment monitoring his breathing overnight was turned off for almost three hours.
James needed the equipment as he suffered from sleep apnoea, which can cause life-threatening stoppages in breathing.
He had numerous health conditions, including having to be fed through a tube, bowel problems, epilepsy and hearing and sight impairment, but was described by his family as a "cheeky, funny, happy and sometimes mischievous boy".
James had gone to the Portland Hospital for an operation to create a colostomy bag so he could prepare for moving into a new class at school.
The operation was a success and James managed to speak to his brothers and sisters on Facetime.
But a few hours later, he was found unresponsive in his bed at the private hospital's paediatric intensive care unit.
Eight staff, a mixture of employees and agency staff, were caring for six children overnight when the incident happened on August 25 last year.
Hudgell solicitors, which is representing the family, said staff have since given conflicting accounts of what happened.
The serious incident report by the hospital shows staff took the decision to remove the monitoring equipment at 1.15am without consulting a more senior member of staff.
It said staff also had breaks during the night shift which were longer than hospital rules permitted. They also failed to formally complete and document handovers of their responsibilities.
The investigation found failures in record-keeping, with failure to document observations of James throughout the night.
Staff also immediately turned off sounding alarms when they reconnected James' monitoring equipment at 3.58am.
There is also uncertainty over how long it took staff to call for help.
The report concluded that the root cause of James' death "appears to be the removal of vital signs monitoring between 1.15am and 3.58am, as this resulted in the inability to be alerted to a suspected deterioration in the child's respiratory condition and reducing oxygen saturations."
On the night James died, his father, John, had left his beside in the early hours.
But just hours later, he and his wife Marguerite were told their son was unlikely to recover.
James was transferred to the Treehouse Hospice in Suffolk, where he died the following day.
James' family are now launching legal action against HCA Healthcare UK, which runs the Portland Hospital.
Mrs Dwerryhouse said: "It has been completely heart-breaking. James had health issues and needed a lot of health support in his life, but he was full of life, cheeky, funny, happy and occasionally mischievous, like all little boys.
"Only days earlier, he'd been to a summer club and he'd been chasing around, up and down the slides and having a great time. He wasn't a fragile boy at all, he wasn't poorly and he certainly wasn't a boy who was at risk of dying.
"Everybody loved him, from his school to those who supported his health. This was a routine operation, and the operation itself was a complete success.
"To then lose him simply because hospital staff haven't cared for him and checked on him as they should is something we cannot accept and never will."
Solicitor Rosamund Rhodes-Kemp said an inquest is still to be held and there are many questions which need serious investigation.
"What cannot be disputed in this case is that a huge, unforgivable error was made in that James' monitoring equipment was turned off when it should have remained on at all times," she said.
"Conflicting accounts have been provided as to what exactly happened when James was found not breathing.
"This means it has yet to be made clear whether the alarm was raised immediately, or whether there was a 20-minute delay, which obviously could have had a big impact on James's chances of survival."
A spokesman for the Portland said: "We send our deepest sympathy to the family of James Dwerryhouse.
"We are co-operating with all investigations, and we are unable to comment further until they are completed."
A statement from Scotland Yard said: "The boy was admitted to a central London hospital on Wednesday August 24. He died on August 28.
"The death was passed onto the Westminster Coroner and Westminster CID officers are undertaking inquiries on behalf of the coroner.
"The death is deemed to be non-suspicious. No one has been arrested."