The Heath Hospital has hit the highest level of escalation as emergency department staff struggle to cope with "extreme pressure" on services.
Cardiff and Vale University Health Board has placed its University Hospital of Wales and University Hospital Llandough sites on level 'red' after patient wait times hit more than 12 hours.
It means both intensive care and critical care units are now full and A&E has reached its full capacity.
Cardiff's level 'red' came less than a day after neighbouring health board Aneurin Bevan issued an urgent appeal with A&E waiting times hitting 'record levels' at the Grange Hospital.
In a statement, the health board said: "We are experiencing significant pressure across our Health System, with our University Hospital of Wales and University Hospital Llandough sites operating at Escalation Level 4, denoting extreme pressure.
"Our Emergency Unit at UHW is extremely busy and we would urge you to only attend the department where absolutely necessary - if you have a life-threatening illness or serious injury."
What does Escalation Level 4 'red' mean?
The emergency response to hospital pressure is triggered when the following has occurred:
Emergency admissions have significantly exceeded predicted levels and available capacity
Waiting times of more than 12 hours have occurred
A&E capacity is unable to meet further demand
It takes more than 60 minutes for ambulance patients to be transferred to hospital care
Patients are waiting more than four hours for first contact with an assessing clinician
No transfers or discharges are taking place
No critical care unit (CCU) or intensive care unit (ITU) capacity available
All planned admissions cancelled
No divert to neighbouring health boards in place
Cardiff and Vale University Health Board is asking patients to consider all of the options for accessing healthcare and has put out the following advice:
"If you are unwell and think you need to be seen urgently in either our Emergency Unit or Minor Injuries Unit, please phone first by calling NHS 111. They will assess you and book you an appointment if necessary.
"If you are unwell and unsure what to do, you can visit the online symptom checker or call NHS 111 for advice. Our Primary Choice webpages also highlight the range of services available through Primary Care providers such as GP practices and community pharmacists.
"We are also asking those with loved ones who are in hospital and are well enough to be discharged to support their discharge home. This is best for your loved ones and it will also help free up beds for those most in need.
"Please help us to deliver services safely by only using our services as absolutely necessary, and sharing this information with friends and family, thank you."