Many calls to the Welsh Ambulance Service will not be responded to when workers who are members of the GMB union strike on Wednesday, ITV Wales understands.
Paramedics, emergency medical technicians and some call handlers are among almost 1,500 workers in Wales expected to walk out in a dispute over pay and working conditions.
A number of services will be protected, with some striking workers set to respond to the most serious, life-threatening calls from the picket line.
But with almost a quarter of the workforce in Wales members of the union, the industrial action is likely to cause major disruption with fears lives will be put at risk.
The Welsh Ambulance Service usually responds to more than 1,800 emergency calls a day, but its ability to attend incidents has been increasingly hampered by long hospital handover delays and staff sickness.
'The members do not want to strike, they are doing this because they are not being given any other option', says Nathan Holman, GMB
The strike is set to begin at midnight on Tuesday and last for 24 hours, with its impact expected to be felt in the following days. Another walkout is due to take place on 28 December.
Welsh ambulance workers are joined by around 8,500 staff members in eight other trusts in England who voted to strike.
There are also concerns over the possibility of an even greater number of workers taking sympathetic action.
Discussions over derogations are still ongoing between the union and the Welsh Ambulance Service, but 'red', the most immediately life-threatening, and some 'amber 1' calls are set to be included.
Red calls include cardiac arrests, continuous fitters and people who are unconscious or not breathing and account for around 15% of calls to the service every day.
Amber 1 calls are those deemed serious but not immediately life-threatening and include strokes, heart attacks and serious car crashes.
Control rooms are also set to be exempt from the strike - but that relies on staff choosing to participate in derogated activity.
The ambulance service is also responsible for some hospital transfers, but only a limited number of these are likely to go ahead including for oncology and dialysis appointments.
The Welsh Ambulance Service does not have plans to draft in the army for support as it did during the Covid pandemic.
This is due to the fact the military personnel who were used then would not be available now and the service would have to train new people, which would put extra pressure on non-striking workers.
But the UK Government announced it is deploying 1,200 troops from the army, navy and RAF to fill in for ambulance drivers and border staff during walkouts in England.
The Welsh Government announced earlier this year that most NHS staff would be entitled to a pay increase of £1,400, amounting to an uplift of 7.5% for lower paid staff in bands 1-4, and an increase of 4% for staff in bands 6-7.
The Welsh Government has said it is limited by the funding it gets from Westminster, but the UK Government said Wales will see its budget increase by £1.2 billion over the next two years.
The Welsh Government said it is having regular meetings with the trade unions.
Unions in England will attend 11th-hour talks convened by UK Health Secretary Steve Barclay but the discussions are not expected to avert the action.
Rachel Harrison, GMB National Secretary, said: "This is as much about unsafe staffing levels and patient safety as it is about pay.
"A third of GMB ambulance workers think delays they’ve been involved with have led to the death of a patient.
"Something has to change or the service as we know it will collapse."