Last month saw the highest number of people visiting A&E for any December on record, figures released by the Welsh Government have revealed.
The performance for 95% of patients to be seen within the target of four hours was the lowest since March 2016.
Statistics published today also show those aged 85 or over attending A&E was the highest it has ever been.
The figures come as a number of Consultants in Wales wrote to the First Minister warning him they "are severely and chronically under-resourced" with "neither sufficient staff, nor sufficient beds."
The Welsh Ambulance Service said it saw a 170% increase in calls over the first four hours of New Year's Day compared to a normal day.
The average number of patients visiting A&E in December was 2,656 - 3.8% lower than the month before - but 5.4% higher than December 2016.
Average number of patients visiting A&E in December (5.4% higher than Dec 2016)
Patients spent 12 hours or more in A&E in December - an increase of 599 patients from November
There was an increase in the number of calls to the ambulance service - of 10.7% from the previous month.
Average emergency calls to the ambulance services
70% of emergency responses to 'immediately life threatening calls arrived within eight minutes which is above the target of 65% - but down from 73.3% in November 2017.
Despite the busiest December on record for our A&E Departments and the busiest ever month for the Welsh Ambulance Service, our NHS has continued to perform under exceptional pressure in delivering emergency and scheduled care. I want to once again thank our NHS and social care staff for their commitment and hard work in delivering care during this extremely busy period. We have worked closely with health boards and partners since last winter to plan for this winter period and invested an additional £60m to help them deliver urgent and emergency care, and ensure planned care activity can continue during this time. We still need to drive up performance in a number of areas but under severe pressure and record levels of demand our NHS has held up and continues to deliver high quality healthcare to patients.
"We are severely and chronically under-resourced"
Emergency hospital consultants have warned safety is currently being compromised in "all departments" to an "unacceptable degree."
In a letter by 46 Emergency Department physicians and consultants it says they felt "compelled...to speak out in support of staff...and to share the very serious concerns we have for the safety of our patients."
The consultants warned planning ahead of the winter have "fallen well short of what was required to maintain adequate care."
Safety is currently compromised in all of our departments to an unacceptable degree. This is despite the enormous efforts made by our staff to keep patients safe in extremely challenging circumstances.
The letter also highlighted some personal experiences from doctors and medical staff:
More patients in the ED waiting for ward beds than the ED can actually accommodate, with no space to see any newly presenting patients
Staff arriving for shifts to find the patients they cared for the previous day (and often the day before) still in the ED
Makeshift arrangements made to accommodate extra patients in hospitals, often with suboptimal staffing
Multiple ED staff in tears at work as they do not feel they can deliver the care their patients need
EDs in the precarious position of having nowhere to treat a newly presenting critically ill patient
Multiple ambulances waiting outside every ED, with a knock-on effect on WAST’s service delivery to the Welsh public
We ask, as a matter of urgency, that you consider supporting strategies that will reduce crowding in our Emergency Departments. These include: