The University Hospital of Wales is urging people to people to only attend its accident and emergency unit if it is "life or limb-threatening."
It says it is "under extreme pressure and is exceptionally busy at the moment."
In a statement, it added waiting times are currently more than nine hours which is "far longer than we would like."
Cardiff and Vale University Health Board is urging people to call the NHS Wales symptom checker on 111 or to visit your community pharmacy who can provide advice on a number of minor illnesses.
It thanked people for their "help and understanding."
"We would like to thank people across Cardiff and Vale of Glamorgan for their help and understanding during this time of extreme pressure in our health service."