South Western Ambulance Service has declared a critical incident due to "extreme pressures" affecting the trust's 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 in the region.
The incident follows the same declaration by North East Ambulance Service on Tuesday due to "unprecedented" pressure after the holidays.
The North East Ambulance Trust declared a critical incident for the second time in nine days due to significant delays for more than 100 patients waiting for an ambulance, as well as a reduction in ambulance crew availability to respond because of delays in handing over patients at the region's hospitals.
Critical incident status means that ambulance trusts cannot provide usual critical services and patients may face harm.
Wayne Darch, deputy director of operations for the South Western Ambulance Service, said: "Please help us to help the patients who need us most by only calling 999 if a patient has a life-threatening condition or illness.
"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.
"We are doing all we can to manage these winter pressures and we are sorry that we are unable to respond to some patients as quickly as we would like. We will get to you as soon as we can.
"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."
It comes after the majority of ambulance services in England declared critical incidents last week ahead of strikes and the Christmas period.
The GMB union announced in early December that more than 10,000 ambulance workers across nine trusts in England and Wales would walk out on December 21 and 28.
The union postponed and re-scheduled the industrial action planned for Wednesday December 28 to January 11 next year as a "thank you to the public" for their "incredible support", according to a GMB statement.
Hospital Trusts in England have also declared critical incidents on Wednesday.
Sandwell and West Birmingham NHS Trust said it was seeing "immense pressure" on its services and has had to declare a critical incident due to "exceptionally high numbers" of patients awaiting treatment in its A&E departments.
Surrey and Sussex Healthcare NHS Trust said it has taken the same action after seeing "record numbers" of patients in A&E, calling 999 and 111, and accessing GP services.
Only Dial 999 if It Is 'List-Threatening'
Many ambulance services and hospital trusts across the UK have urged members of the public on their websites and Twitters to only dial 999 in "list-threatening" emergencies, and encouraged people to call 111, contact their GP or visit their local pharmacists.
East of England Ambulance Service has declared a business continuity incident to manage "extreme demand".
The trust declared the incident on Wednesday evening, for the second time this month, due to high call volumes and ambulance delays outside hospitals.
It also called a critical incident between December 19 and 21. Declaring a business continuity incident ensures resources are focused on patients with the greatest need and increases access to wider support from health and care partners.
It means the service can ask NHS colleagues to make releasing its crews a priority, can prioritise the sickest patients and those unable to make their own way to hospital, and can cancel some staff meetings, training and assessment.
In a statement on its website, the trust said: "Our staff continue to work incredibly hard in challenging circumstances, to respond to calls and incidents as quickly as possible.
"If you need to contact us because of a life-threatening condition or serious injury, then call 999. For everything else, we would urge you to please use 111 online, speak to your GP or use a minor injuries centre."
On Twitter, the ambulance service said it was under "extreme pressure" as it announced the raised status.
This article contains information from PA Media.