Wales’ biggest hospital is experiencing “unprecedented levels of pressure” ahead of the bank holiday weekend.
The University Hospital of Wales (UHW) in Cardiff said the spike in demand is resulting in significant waiting times at its emergency department.
It said there has been an increase in patients attending A&E with long-standing health issues that can be managed elsewhere.
The hospital has issued a plea for the public to only attend in an emergency.
The extraordinary pressures comes ahead of the three-day bank holiday to celebrate the Coronation of King Charles III, with NHS often experiencing increased demand during public holidays.
A UHW spokesperson said: “We would urge the public to continue to help our team by choosing the right service for your needs so our emergency unit can support those most in need of emergency care.”
They added: “By accessing the most appropriate service you will support our teams to prioritise and care for the most unwell patients who need us most.”
Cardiff and Vale University Health Board said it has not declared a critical incident at the hospital.