The East of England Ambulance Service has declared a critical incident status amid a high volume of 999 calls and hospital handover delays.
By declaring a critical incident, NHS services in the region are focused on patients with the greatest need and can access wider support from health and care partners.
A spokesman for the ambulance service said: "Staff will continue to work hard in challenging circumstances and respond to incidents as quickly as possible.
"If people need to contact the ambulance service due to a life-threatening condition or serious injury, they should still call 999.
"For everything else, people are encouraged to use 111 online, speak to a GP or use a minor injuries centre."
It comes as thousands of nurses are staging a walkout in a row over pay as a series of strikes hit the UK in the run-up to Christmas.
Around a quarter of hospitals and community teams in England are taking part in the strike, alongside all trusts in Northern Ireland and all but one health board in Wales.
Nurses in Norfolk and Suffolk will not take part in the first round of strike action.
The NHS Trusts and Integrated Care Boards (ICB) affected by the strike action in the Eastern region today (December 20) are Cambridge University Hospitals, Cambridgeshire and Peterborough, Cambridgeshire Community Services, Hertfordshire Community and NHS Hertfordshire and West Essex.
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