South Western Ambulance Service has declared a critical incident due to "extreme pressures" affecting its ability to respond to patients following the Christmas break.
As of 11.30am on Wednesday, 482 patients were waiting for ambulances across the South West, with 106 patients awaiting handover at hospitals.
Declaring a critical incident allows trusts to prioritise the patients most in need and to instigate additional measures to protect patient safety.
Yesterday, North East Ambulance Service also declared a critical incident for the second time in just over a week due to "unprecedented" pressure following the Christmas break.
The majority of ambulance services in England declared critical incidents on 20 December ahead of strikes over the Christmas period.
South Western Ambulance Service covers 10,000 square miles, including Bristol and counties such as Devon, Dorset, Gloucestershire, Cornwall and Somerset.
People have been urged to only call 999 if someone's life is in danger and in other cases call 111 or their GP.
"If the condition of a patient is not life-threatening we may direct them to an alternative service. So please help us by accessing the right service for the care you need," said deputy director of operations Wayne Darch.
"Please do not call back simply to ask for an estimated time of arrival of an ambulance. We cannot provide one, and it blocks our lines for other callers," he added.
About 25,000 ambulance workers went on strike on 21 December and two further strike days have been announced for England in January amid a dispute over pay and staffing.
Two hospital trusts also declared critical incidents on Wednesday.
Sandwell and West Birmingham NHS Trust said there was "immense pressure" and "exceptionally high numbers" of people waiting for treatment in A&E departments.
Surrey and Sussex Healthcare NHS Trust took the same action due to "record numbers" at A&E, calling 999 and 111 and accessing GP services.