The Welsh Ambulance Service has faced 'extreme pressure' - the extent of which some staff say they've never seen before, ITV Wales has been told.
The trust has confirmed it had to implement its highest level of escalation - level 4 - yesterday in order to provide 'the best possible service' to patients.
But one ambulance crew member told ITV Wales that under the level 4 they believed the service was "unsafe." They said it was "all due to hospitals having no beds [space]."
One paramedic with years of experience in the Trust said: "That's the first REAP [level] 4 to my knowledge."
In a sign of how busy the service is, they added: "I have heard of staff going to calls that are days old, not just hours. You wouldn't want to be ill."
Responding to those concerns, the service said: "As part of the NHS in Wales we always endeavour to provide the best level of service possible.
"The Trust is working hard with colleagues across NHS Wales to ensure that we are able to respond to those most in need of our help as soon as possible.
"Inevitably, this will mean it may take longer than we would like for us to get to some patients with less serious conditions."
According to an internal Ambulance Service document seen by ITV Wales, the level of escalation in place yesterday is only triggered, among other things, when hospital handover times exceed 300hrs daily over seven days.
The Trust said: "The reason we went into Level 4 yesterday was the level of handover delays, the subsequent effect on ambulance availability and the increase in emergency activity."
ITV Wales understands that when level 4 escalation is reached, the trust's own framework dictates that it should delay allocating ambulances to low-level non-emergency calls, draft in all available staff and use taxis to get some patients to hospitals.
The ambulance service said: "Levels are reviewed on an ongoing basis and therefore fluctuate in response to demand for services. Today we are at Level 3, which is not unusual during busy periods."
But it's not just the ambulance service facing huge demand.
Health boards across Wales are reporting extremely busy emergency departments and some have been pleading with the public to come and pick up relatives who are ready to leave hospital just so that space can be freed up inside.
ITV Wales has also been told that last night, the Princess of Wales hospital in Bridgend and Morriston hospital in Swansea were asking other hospitals in south Wales what space they had to take patients.
But despite calling for assistance from other hospitals, Abertawe Bro Morganwwg University Health board says no patients or ambulances were turned away at the front door.
In a statement, it said: "The Princess of Wales Hospital was extremely busy overnight on Tuesday, in the wake of the recent severe weather. However, we had no pre-agreed diverts in place on that night.
"All ambulances destined for the Princess of Wales Hospital, that we are aware of, arrived here and the patients were accepted."
The message from health boards and the Welsh Government to the public is to 'choose well' and only dial 999 if what they are experiencing is a genuine emergency.
Advice from the ambulance service includes:
Ensure you have plenty of basic first aid supplies and/or any prescription medicines which you need
999 and Emergency Departments are for the sickest patients only. Ambulances are a clinical service not a transport service – our job is not to take you to hospital if you are well enough to travel safely yourself or with a friend/family member
If you feel unwell and are unsure what to do, please check out the online symptom checkers at . You can also call the service on 0845 46 47 for 24/7 health advice and information or 111 if you live in Swansea, Bridgend, Neath Port Talbot or Carmarthenshire.
There’s also local pharmacies, which are a great source of advice and low cost over-the-counter remedies for many common ailments and symptoms, and Minor Injuries Units, where there is no need for an appointment.