Medical director Sir Stephen Powis said people should still use 999 for emergencies and NHS 111 for urgent needs.
But planned hospital care, including operations and appointments, will face major disruption, with many patients facing cancellations.
“This new round of strike action will again hit the NHS very hard, with almost all routine care likely to be affected and services put under significant pressure,” Sir Stephen said.
“As ever, we are working to ensure urgent and emergency care is prioritised for patients.
“GP services and pharmacies are also available for patients and can be accessed in the normal way.
“Patients who haven’t been contacted or informed that their planned appointments has been postponed are also urged to attend.”
The hot weather was likely to cause additional pressure, Sir Stephen added.
