The number of patients spending more than 12 hours in A&E was the highest on record in January, new statistics show.
The performance for 95% of patients to be seen within the target of four hours fell to 78% - the lowest performance since March 2016.
January also saw the highest number of people attending A&E with the average at 2,615 across Wales - a total of more than 80,000.
This is up 5.7% on the year to January 2014.
The figures also showed those aged 85 or over attending A&E was the second highest it has ever been, with flu adding to pressure on emergency departments.
The Royal College of Emergency Medicine in Wales said the rise in figures for patients waiting 12 hours or more is "catastrophic" and "unacceptable".
This catastrophic rise is unacceptable and shocking.
Flu has also had an impact on capacity across our system with the highest rates for six years. During this busy time, our NHS and social care staff have continued to deliver emergency and planned care with compassion and professionalism. I would like to thank them for their exceptional commitment and hard work during this extremely challenging period. Despite the increased workload, their second highest month of activity on record, the Welsh ambulance service maintained its record of meeting the national target for responding to red calls in every month since the new response model began. We will continue to respond to areas of performance that need to be improved but despite record levels of demand on service, our staff have provided a resilient and professional response to support patients this winter.
Calls were made per day to the ambulance service
Emergency calls made to the ambulance service in January
69.7% of emergency responses to 'red calls' - immediately life threatening calls - arrived within eight minutes which is above the target of 65%, but down from 70% in December 2017.
The Welsh Government says there was an average of 3,602 outpatient referrals per working day.
These figures however only includes data from the six health boards due to 'data quality issues' with Betsi Cadwaladr University Health.
Targets were also missed for cancer treatment times.
Currently, patients with cancer are split into two distinct groups:
Those referred via the 'urgent suspected cancer route' which includes patients referred by their GP to a hospital as urgent with suspected cancer.
Those not referred via the urgent suspected cancer route which includes patients with cancer not already included as an urgent suspected cancer referral
87.2% of patients who were newly diagnosed with cancer started 'definitive treatment' within the target time of 62 days in December - which is below the target of 95%.
96.8% of patients newly diagnosed with cancer not via the urgent route started treatment within the target time of 31 days in December - which is below the target of 98%.