Five deaths have been linked to problems with the 111 helpline in the last five months, according to NHS information.
Incident logs submitted to NHS England and released to The Daily Telegraph through a Freedom of Information (FOI) request show that more than one patient has died after call handlers entered the wrong information into a system used to assess a caller's condition.
The NHS has insisted that all the incidents were fully investigated and "lessons learned".
The 111 helpline is recommended for non-emergency medical incidents and callers speak to a "highly-trained adviser, supported by healthcare professionals".
Call handlers "ask a series of questions to assess your symptoms and immediately direct you to the best medical care for you," according to the NHS.
But it has previously come under fire for missing symptoms.
In February, Melissa Mead met Jeremy Hunt after a 111 call handler failed to spot her one-year-old son William had sepsis caused by an underlying chest infection and pneumonia.
The call centres are run either by an ambulance trust, a private firm or a social enterprise and call handlers use a system called Pathways to classify callers symptoms.
The FOI request revealed that one call handler put the wrong answer into the system when dealing with a patient in NHS England's South region.
A week after contacting the helpline, the patient died from severe respiratory distress.
A patient who died in the Midlands and East region had called 111 with shortness of breath.
The incident report said that "ineffective triaging" had seen the call handler focus on the patient's diabetes over the breathlessness.
Another patient in the same region died when a call handler chose the wrong option and so Pathways did not prompt the handler to ask the caller to perform CPR.
The Daily Telegraph reported that the NHS would only release data for five months as any more would cost more than the limit for FOI requests.