A 96-hour junior doctors' strike is set to cause "significant disruption" at hospitals such as Stevenage's Lister and Watford General.
The industrial action will take place from 7am on Friday (August 11) until 7am next Tuesday (August 15).
Princess Alexandra Hospital in Harlow, Lister Hospital in Stevenage and Watford General Hospital are all likely to be affected.
With hospitals "focussed on protecting services for life-threatening emergencies", the public have been reminded to only attend accident and emergency departments (A&E) if they need treatment for a very serious illness or injury.
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August is the ninth month of strikes across the NHS, with approximately 778,000 hospital appointments impacted.
According to the NHS Hertfordshire and West Essex Integrated Care Board, previous strikes have seen 7,000 to 20,000 junior doctors off per day.
Dr Jane Halpin, the board's chief executive, said: "This further strike action over a weekend will present many challenges to patients, our hospitals, and some of our GP practices too.
"With large numbers of doctors expected to be absent, hospitals will not be able to run as normal and services will be working under immense pressure.
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“Even with the careful planning that is taking place, both routine and emergency care are likely to be significantly affected.
"In order to prioritise life-saving emergency treatment for people who need it, hospitals will postpone some non-urgent operations, treatments and routine appointments.
"Where this is the case, patients will be contacted directly.
"This is really unfortunate for those whose treatment is delayed, but with the majority of doctors not working, remaining staff must cover the essential care patients need.
“A&E is always for serious injuries and life-threatening emergencies only, so in a genuine medical emergency always call 999.
"If you call 999 for an ambulance when you don’t need one, you could endanger the life of another person in desperate need of emergency care.
"We have more staff in the NHS 111 service to cover this challenging week, so please use 111 online or by phone if you cannot manage symptoms yourself at home.
"I would like to thank everyone for their understanding and support at this challenging time.”
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