An agency nurse working for the NHS was filmed switching off her patient's life support machine by mistake.
Tetraplegic Jamie Merrett, 37, had a bedside camera set up at his home in Wiltshire, after becoming concerned about the care he was receiving.
Within days, it captured the moment Violetta Aylward switched off the ventilator, leaving him brain-damaged.
Ambition 24hours, which supplied her, said it could not comment as an internal investigation was continuing.
A confidential report by Wiltshire social services into the incident - leaked to the BBC's Inside Out programme - concluded the agency was fully aware it was required to supply a nurse with training in the use of a ventilator, but the company did not have adequate systems in place to check what training their staff had received.
Mr Merrett, from Devizes, has been cared for at home on a life-support machine since 2002 after being left paralysed from the neck downwards following a road accident.
Despite his disabilities, he was able to talk, use a wheelchair and operate a computer using voice-activated technology.
His sister Karren Reynolds said he had become increasingly worried about serious errors involving nurses operating his ventilator, but claimed that health bosses did not act on e-mails of concern which he sent them.
In January 2009, he arranged to have a camera installed in his room. A few days later, the ventilator was switched off.
After 21 minutes, the machine was eventually restarted by paramedics but by then Mr Merrett had suffered serious brain damage.
Ms Reynolds, who is considering legal action, said his level of understanding had dropped to that of a young child.
"His life is completely changed. He doesn't have a life now," she said.
"He has an existence but it's nowhere near what it was before. He is very brain damaged compared to what he was before. He was a highly intelligent man and you could have long in-depth conversations with him and now it tends to be more simplistic."
The solicitor acting for Mr Merrett, Seamus Edney of SJ Edney in Swindon, said: "In my experience, this is the worst case of negligence on the part of a nurse.
"No-one has come forward to make any admission, so now almost two years after the event we are trying to get someone to admit liability for what has happened."
The NHS Wiltshire Primary Care Trust said in a statement: "[We have] put in place a series of actions to ensure that such an event will not occur again either for this patient or others. The incident is the subject of likely litigation so the PCT is restricted in what further it may say in public."
Ms Aylward has been suspended while the incident is investigated by the Nursing and Midwifery Council.
Their guidelines say a nurse should work within their level of competence and have the skills to undertake whatever care they are delivering.
Ms Aylward, who is from Reading, has not responded to requests for an interview.
The programme will be shown on Inside Out West and Inside Out South on BBC One at 1930 BST on Monday 25 October.