On-call doctors at an NHS 111 helpline at the centre of failings that led to the death of a baby were so "overworked" some had fallen asleep on duty, a whistleblower has claimed.
Sarah Hayes, a former senior call adviser for the non-emergency hotline in the South West, told the Daily Mail she believed the service was "unsafe" for young children and babies.
She worked at the same service as a call handler who failed to recognise William Mead had sepsis caused by an underlying chest infection and pneumonia before the 12-month-old died in December 2014.
Jeremy Hunt apologised to the child's family, saying it was let down in the "worst possible way" after details of a string of NHS failings emerged in a report last month.
William's mother Melissa, 29, from Penryn, Cornwall, has called for those who run the 111 NHS helpline to only allow doctors and nurses to handle calls involving young children.
Call handlers on 111 are not medically trained and follow a set series of questions to identify patients who need further help.
Ms Hayes told the paper she felt she had to speak out after William's death.
Writing in the paper, she claimed: "The service at the moment is unsafe, particularly for children and babies. I don't want more children put at risk like William was."
Describing working conditions at the South West helpline, run by South Western Ambulance Service NHS trust, she said there was "frequently" no on-call clinician in the call centre.
"The nurses and paramedics we did have were so exhausted and overworked that some would fall asleep on shift. I was angry, of course, but I don't feel it was their fault. Put simply, they were exhausted," she said.
"I think anyone with experience of 111 would say it has problems for young babies, and it's really hard to get a good assessment done. I think that passing a young baby to a clinician would be a really good idea but you would need many, many more clinicians to make it work."
Ms Hayes added some callers would wait more than 12 hours for a call back from a medically-trained member of staff and at night up to 65 patients could be on the waiting list.
She claimed tried to raise the issues but said "life was made difficult" and sometimes she was "simply ignored".
According to a report by NHS England, Mrs Mead spoke to medics at least nine times in the 11 weeks leading up to William's death. He was seen by several GPs who failed to spot that his condition was deteriorating.
On the day before his death, Mrs Mead called 111 for advice and also spoke to an out-of-hours GP who did not have access to any of her son's medical records.
The 111 call handler failed to explore further some of Mrs Mead's comments about William's condition, including that his temperature had gone from a high 40C (104F) to a low 35C (95F) - a sign of sepsis.
But the report also blamed GPs for the baby's death, saying a "significant missed opportunity was the fact that the underlying pathology, a chest infection and the pneumonia in the last six to eight weeks or so of William's life, were not recognised and treated".