Patient safety cannot be guaranteed during today's strike by ambulance workers in England and Wales, health bosses have warned.
But NHS England says emergency care will continue to be provided.
People are being asked to use services "wisely" and only call 999 in a life-threatening emergency - with 111 online the first port of call.
Ambulance response times are already twice as long as two years ago and there is a huge strain on A&Es.
In England, eight out of the 10 major ambulance services have declared critical incidents - a sign of the intense pressure they are already under.
Ministers have urged the public to take extra care and suggested they avoid contact sports and unnecessary car journeys.
Unions say life-threatening callouts will continue to be responded to over the next 24 hours but some urgent calls, for example for late-stage labour or a fall in the home, might not be answered.
No industrial action is taking place in Northern Ireland and Scotland, and there will be no strikes in the east of England or the Isle of Wight.
But elsewhere, there is likely to be major disruption as paramedics, call handlers, emergency care assistants and technicians go on strike.
About 750 armed forces staff are being drafted in to cover the walkouts, however their role will be limited. They will not be sent on call-outs involving critical care, nor will they provide any clinical care.
In the run-up to the strike, rhetoric from both sides has intensified.
Writing in the Daily Telegraph, Health Secretary Steve Barclay said: "We now know that the NHS contingency plans will not cover all 999 calls. Ambulance unions have made a conscious choice to inflict harm on patients."
But Rachel Harrison, national secretary of the GMB Union, accused the government of insulting NHS staff by claiming they are making a conscious choice to inflict harm on patients.
Union members had been forced into action and did "not taken the decision lightly", she told BBC Breakfast.
She said earlier that ambulance staff were tired of spending all day outside a hospital with a patient because of delays in handing over patients to A&E. They often did not know whether patients would "still be alive" when they reached them after a callout.
"We've been raising these issues for years and [have] been ignored," she added.
In a letter to the prime minister, the NHS Confederation, which represents healthcare systems in England, Wales and Northern Ireland, said there was now "deep worry among NHS leaders about the level of harm and risk that could occur to patients".
"This is not something NHS leaders would ever say lightly," the letter stated, "but many now tell us that they cannot guarantee patient safety".
The letter has now also been signed by NHS Providers, the body which represents hospital trusts, mental health and ambulance services.
The industrial action by ambulance workers follows two days of strikes by nurses this month over pay. It has also been one of the busiest months on record for people attending Accident and Emergency departments.
Long waits for ambulances after an emergency 999 call have become a regular occurrence, as have queues of ambulances outside A&E waiting to offload patients.
The strikes have come "at the worst possible time", Dr Adrian Boyle, President of the Royal College of Emergency Medicine, told Radio 4's Today
Covid, the strep A outbreak and rising flu cases are among the reasons the system is under pressure, "regardless of industrial action", he said.
"It's already very difficult, we're seeing deaths because of delay and dilution of care anyway."
Ambulance workers are asking for a pay rise above inflation - although not a precise figure - and a plan for recouping lost earnings over many years.
Mr Barclay met union representatives on Tuesday afternoon but there were no discussions around pay - only what care would still be provided during the strike.
Onay Kasab, from Unite, said the meeting was "entirely pointless" because the health secretary refused to discuss pay.
Mr Barclay called the strikes "deeply regrettable" and urged the public to take extra care and check in on vulnerable friends, family and neighbours.
He said most ambulance staff have received a pay rise of at least 4%, taking average earnings to £47,000. A further pay increase would mean taking money from frontline services, he added.
Prime Minister Rishi Sunak has insisted he will not back down against striking workers. He has said the best way to help the workforce would be to reduce inflation as quickly as possible.
Not all Conservatives agree and want to see some flexibility from the government. Sir Geoffrey Clifton-Brown, MP for the Cotswolds, said: "These things have got to be solved by negotiation. There are people who are desperately likely to need this service over Christmas."
NHS England says it will have more staff answering 999 calls and is helping individual trusts speed up the process of handing over emergency patients and discharging those well enough to go home.
London Ambulance Service will not dispatch an ambulance to all 999 callouts. Instead, a team of clinicians will call patients back to see if they can be helped in other ways.
They expect there to be 200 ambulances compared to the normal 400. Most will be staffed by the military, with a clinician alongside that may or may not be a paramedic. Taxis may also be used for some patients.
If there are not enough ambulances to get to all life-threatening emergencies, staff will leave the picket line to respond, the head of the service, Daniel Elkeles, said.
On Tuesday, eight ambulance services declared critical incidents, including North East Ambulance Service, South East Coast Ambulance Service, the East of England Ambulance Service, Yorkshire Ambulance Service and South Central Ambulance Service, because of pressure on services.
A critical incident allows services to prioritise certain patients and cancel non-urgent demands on staff such as training. It can happen because of a very high number of calls, for example.
Source: BBC
Image: Getty Images
