The region's ambulance service has moved to the "highest escalation level" due to a "huge spike" in calls.
The East of England Ambulance Service (EEAST) has moved to REAP (Resource Escalation Action Plan) Level 4 in response to sustained high demand on the service and handover delays at hospitals.
This means it can put in additional measures to get to patients as quickly as possible, such as deploying all available clinicians to frontline duties.
In the week ending November 30, the EEAST took almost 24,000 emergency calls from the public – an increase of nearly 3,000 calls on the previous week.
Hospitals have also faced increased pressure due to rises in respiratory and viral illnesses.
Neil Moloney, chief executive of EEAST, said: "There is an extraordinary level of pressure on the health system nationally, and we have made the decision to move to REAP 4 – the highest escalation level.
"We have seen a huge spike in demand, driven by an increase in respiratory illness, which means many hospitals are at capacity and our ambulance resources are stretched as a result.
"I urge the public to think carefully before using 999.
"If you have a medical emergency, we will get to you, but there may be a delay.
"During periods of high demand, patients with immediately life-threatening conditions are our priority.
"Those with less serious conditions will experience delays and may be referred to other NHS services or given self-care advice.
"We apologise to anyone affected by these delays."
People are being urged to use NHS 111 if they do not need emergency care and ensure they are up to date with vaccinations.
In cases where it is safe to do so, those who call 999 may be asked to make their own way to hospital, freeing up ambulances to attend to the most unwell patients.
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