Paramedics on strike across the South West are being called off the picket line to deal with the most urgent calls.
Ambulance workers at South Western Ambulance Service Foundation Trust (SWASFT) are among those across the UK who have walked out today in a row over pay and working conditions.
But the ambulance service has confirmed it had an agreement with the Unison union, which means its members will stop striking to attend the most serious callouts during the industrial action.
A SWASFT spokesperson said the service works on a local level with Unison so a derogation agreement can be made.
"It is very well-documented that some of those derogations means that we are in agreement that we're able to call staff off the picket line if needed," the spokesperson said.
They added the callouts were for category 1 and 2 calls - category 1 being the most life-threatening such as cardiac arrest.
Category 2 covers conditions such as stroke, heart attack and sepsis. However strokes fall into the higher end of category 2 cases, so clinicians will determine what response is needed.
SWAST has declared a critical incident due to the ongoing pressures, along with seven other ambulance trusts across the country. This means the trusts are on their highest level of alert.
Will Warrender, chief executive at the service, said: “Please help us to help those patients who need us most by only calling 999 if a patient has a life-threatening condition or illness.
"We know there are patients waiting for an ambulance, and we will get to you as soon as we can. We are sorry that we are unable to respond as quickly as we would like.
“We are doing all we can to manage winter pressures and the upcoming industrial action, including receiving military support during industrial action days which will allow our ambulance clinicians to be manning more ambulances and reach patients more quickly.
"The public can help us by only calling 999 for life-threatening emergencies, we absolutely must have crews available for the most life-threatening conditions and we will be prioritising these patients.
“I’d like to express my huge thanks to our people and NHS partners working to deliver the best possible care they can for our patients over the coming weeks.”
Health Secretary Steve Barclay has today accused unions of making a conscious choice to harm patients. Unite said his comments are a "blatant lie".
He said: "Well it’s the trade unions that have made a decision not to give a national undertaking to cover all of the emergency calls, to leave that to individual decisions with local trusts and with call handlers in terms of some of those emergency calls that will be responded.
"The government has put in place the contingency measures that it can."
The government is refusing to budge on its stance on pay and maintains it will stick by independent pay review bodies' recommendations.