Wales' biggest hospital has issued its highest level of alert after a "significant and sustained adverse demand" on services.
Cardiff and Vale University Health Board said there were particular problems within the Emergency Unit at the University Hospital of Wales.
In a statement, the hospital said the black alert was triggered as a result of winter operational pressures and limited bed availability.
On Tuesday 7 November, a 'Business Continuity Incident' was issued.
Each of Wales' health boards has five levels of escalation which determine how pressured its services are. A 'Business Continuity Incident' is the highest level 5 and is also known as a black alert.
The Health Board stated that "elongated patient stays in hospital" are contributing to the adverse demand on services at the University Hospital of Wales in Cardiff.
It added that it was continuing to "work collaboratively with colleagues from the Local Authorities to support the onward discharge of medically fit patients at home."
During a black alert, the health board diverts staff from non-acute areas of healthcare and enlists the help of more agency staff.
Cardiff and Vale University Health Board has requested people to only attend the Emergency Unit "in cases of a genuine life-threatening emergency which requires immediate medical intervention."
It added: "There has been a recent increase in patients self-presenting with long-standing medical issues that can be effectively managed outside of the emergency unit and/or a hospital setting."
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