The NHS non-emergency helpline is unable to identify when children and babies have potentially deadly illnesses, according to a report.
A system used by the hotline that includes a box-ticking questionnaire by staff who do not have medical training is not "sensitive" enough to identify when children are deteriorating because of the deadly inflammatory condition sepsis, the Daily Mail said.
The report, seen by the newspaper, was carried out by NHS England following the death of a year-old baby following a series of mistakes.
The boy, William Mead, could have been saved if a 111 call handler had realised the gravity of his illness, the report found, and suggested that a medic would most likely have realised the need for "urgent medical attention" if one had taken the call instead of untrained staff using the computer system.
William died from sepsis as a result of a chest infection, and the "root cause analysis report" found 16 mistakes contributed to his death, the Mail said.
Among them was a failure by GPs - who saw him six times in the months leading up to his death - to look for signs of sepsis and give William potentially life-saving antibiotics. They also missed the chest infection which contributed to his death.
The report said doctors were under "constant pressure" not to prescribe antibiotics, even when they believed children needed them, and they were also loath to refer people to A&E units unless "absolutely necessary" because of pressures of patient numbers.
According to the report, doctors working out of hours were also unable to access patients' medical records, the Mail said.
It concluded that a "deteriorating paediatric patient" like William was "not easily identified through the structured questioning", called NHS Pathways, used by the 111 call handlers.
The 111 call by his mother, Melissa Mead, was poorly dealt with by an adviser who would have had no more than a few weeks' training, the Mail said, and who failed to notice the "abnormal behaviour" his mother described.
Most importantly, the report found that even if used properly, the 111 system was "not sensitive enough" to pick up William's illness, a "root cause" in his death.
The report concluded that the tick-box system "lacks sensitivity to the deteriorating paediatric patient".
Mrs Mead, 29, told the Mail: "For us there are no words that can explain the profound loss that we live with after losing William. For us it is a debilitating life sentence that we re-live every day."
She added: "There were multiple occasions in the lead-up to William's death where there were missed opportunities to treat William. We hope those involved in William's treatment will never make the same mistakes again. We also hope the recommendations made improve systems to ensure other lives are not lost."
Apologising to Mr and Mrs Mead, Lindsay Scott, director of nursing with NHS England in the South West, said: "Everyone involved in this report is determined to make sure lessons are learnt from William's death, so other families don't have to go through the same trauma.
"We're particularly grateful to Mr and Mrs Mead for their input and attention to detail, because their experiences during repeated contact with the NHS are critical to our understanding of what went wrong.
"In William's case, it's clear in looking back that opportunities to intervene more decisively were missed, but we will only be able to make a real difference if professionals across the country understand how different choices at critical moments might avert a similar tragedy.
"This report isn't about blame, but about learning and awareness. That applies not just to health staff but to parents as well, because both might be in a position where timely action is crucial."