A major incident declared by The Northern Ireland Ambulance Service will stay in place after only 10% of staff on-duty said they would respond to emergency calls if it was stood down.
The NIAS declared a major incident after levels of cover reached a critical level because of workers going on strike today.
Unions had initially agreed an emergency protocol for ambulance and fire cover during today's industrial action, but the levels of staff at the Belfast Trust who would not be turning up for work forced the move.
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The Ambulance Service said it had no choice but to declare a major incident in order to ensure the public's safety.
On Friday afternoon the NIAS canvassed staff on duty to see if they would remain on duty to respond to Cat A and B calls if the major incident was stood down.
Category A calls are the most serious, where there is an immediate threat to life.
Those which are serious but not life-threatening are prioritised as Category B.
Those which are neither serious nor life-threatening are Category C.
A statement said: "Approximately only 10% of the 41 crews on duty confirmed they were willing to so do. In the absence of a concrete commitment from enough staff required to sustain a safe service, the trust has taken the decision to keep the Major Incident in place."
Hundreds of patients across Northern Ireland who dial 999 could face a long wait for an ambulance because of the 24-hour strike action.
Last-ditch talks between Northern Ireland's health unions and management to agree on a safe level of service for urgent but not immediately life-threatening calls failed yesterday - just hours before the strike began at midnight.
Unions agreed to an emergency protocol for ambulance and fire cover as tens of thousands of health and public sector workers walked out.
They were to respond to immediately life-threatening 999 calls and unions have insisted patients will receive safe care.
The NIAS warned this will lead to a "serious backlog" as Category B and C calls which are deemed not immediately life-threatening will only be dealt with following long delays and said the union action "puts lives at risk".
The Ambulance Service said: "There will then be approximately 330 calls on Friday which will not receive a 999 response and include some stroke patients, elderly patients who have fallen in their homes or on the street, some road traffic collisions, children who have sustained broken bones after falls among many more."Tweets by @TrafficwatchNI
The unions will provide non-emergency transport for palliative care patients, oncology patients, renal patients and paediatric patients.
Patricia McKeown from Unison said: "Where our members are involved we are providing cover for the young, old and infirm in residential care, and at bank holiday levels for community and hospital care.
"On acute care we are providing emergency cover on detailed agreements with the employers."
An estimated 60% of accident and emergency crews and 80% of rapid response paramedics plan to strike.
Paramedics have warned because of the pressures on the system, patients are at risk daily.
One paramedic, who wished to remain anonymous, told the Belfast Telegraph he only recently became supportive of strike action when the pressure became overwhelming.
"I'm going out on strike because we are being sent out to calls exhausted doing a 12-hour shift without a break," he said. "We are burnt-out and at breaking point."
The action that began at midnight has led to seven voluntary ambulances being used to respond to calls across the region.
Unite national officer Kevin McAdam said: "They rightly fear that the scale of the proposed cuts to public services - including to NI Ambulance Service and NI Fire & Rescue Service - will undermine these critical services and threaten public safety.
MLA David McNarry said fears over loss of life were real.
"Mothers, fathers and grandparents can put up with looking after children if the school is closed," he said.
"We can put up with not having a bus, but you can't put up with saying it's OK to put a life at risk.
"There cannot be any fault lines to response times in emergencies. Concerns that lives could be lost are legitimate."
Two of the biggest trade unions involved in bringing major strike disruption to Northern Ireland have yet to reveal how many voted in strike ballots - while a third didn't respond.