NHS England has confirmed that on strike days ambulance services will prioritise clinically urgent cases.
Patients without "life-threatening" emergencies may have to "wait longer than usual" for an ambulance.
NHS Medical Director For Secondary Care Dr Vin Diwakar said: "The message from the NHS to patients is clear – if you need emergency care, please come forward.
"This means continuing to call 999 for life-threatening emergencies as well as using 111 online for other health needs where you will receive clinical advice on the best next steps to take.
"NHS staff have worked hard to minimise disruption but it is inevitable there will be an impact on services".
NHS England is urging the public to continue to access general practice, community pharmacies, and dental services as these are not impacted by strike action. Some patients may be asked "where safe and appropriate" to make their own way to hospital – although they should get medical advice from 111 or 999 before doing so.
Record demand on urgent and emergency care services is already testing resilience with October and November the busiest on record for A&E attendances and the most serious ambulance callouts.
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