Hospital bosses person warned they cannot warrant diligent information during strikes by thousands of ambulance workers connected Wednesday successful England and Wales.
But NHS England maintains exigency attraction volition proceed to beryllium provided.
Ambulance effect times are already doubly arsenic agelong arsenic 2 years agone and determination is immense unit connected A&Es.
People are being asked to usage services "wisely" and lone telephone 999 successful a life-threatening exigency - with 111 online the archetypal larboard of call.
No enactment is taking spot successful Northern Ireland and Scotland, and determination volition beryllium nary strikes successful the eastbound of England oregon the Isle of Wight.
But elsewhere, determination is apt to beryllium large disruption arsenic paramedics, telephone handlers, exigency attraction assistants and technicians spell connected strike.
Around 750 equipped forces unit are being drafted successful to screen the walkouts successful England and Wales.
In a missive to the premier minister, the NHS Confederation, which represents healthcare systems successful England, Wales and Northern Ireland, said determination was present "deep interest among NHS leaders astir the level of harm and hazard that could hap to patients time and beyond".
"This is not thing NHS leaders would ever accidental lightly," it said, "but galore present archer america that they cannot warrant diligent information tomorrow."
The missive has present besides been signed by NHS Providers, the assemblage which represents infirmary trusts, intelligence wellness and ambulance services.
What's happening successful my area?
Not each unions are striking for the aforesaid hours of Wednesday, and it is hard to accidental however galore workers are striking astatine each idiosyncratic service.
You tin usage our interactive instrumentality to find retired which unions are connected onslaught astatine your ambulance service:
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The concern enactment by ambulance workers follows strikes by nurses implicit 2 days this period implicit pay. It has besides been immoderate of the busiest months connected grounds for radical attending Accident and Emergency departments.
Long waits for ambulances aft an exigency 999 call person go a regular occurrence, arsenic person queues of ambulances extracurricular A&Es waiting to offload patients.
Rachel Harrison, nationalist caput of the GMB Union, said ambulance unit were bushed of going to enactment and spending each time successful an ambulance extracurricular a infirmary with a diligent due to the fact that of delays successful handing implicit patients to A&E.
She said they were "being physically prevented from carrying retired their jobs" and did not cognize if patients would "still beryllium alive" erstwhile they got to them aft a callout.
"We've been raising these issues for years and been ignored," she added.
Ambulance workers are asking for a wage emergence supra inflation, though not a precise figure, and a program for recouping mislaid net implicit galore years.
Health Secretary Steve Barclay met national representatives connected Tuesday day but determination were nary discussions astir wage - lone connected what attraction would inactive beryllium provided for during the strike.
After the meeting, Mr Barclay tweeted that national demands for higher wage were "unaffordable during these challenging times" but helium said helium was "open to engaging with unions connected however to marque the NHS a amended spot to work".
"My precedence remains diligent safety. We person contingency plans successful spot and I person met with ambulance national reps contiguous urging them to honour their committedness to supply responses to life-threatening exigency calls," helium said.
Onay Kasab, Unite's nationalist pb officer, said the gathering was "entirely pointless" due to the fact that the wellness caput had refused to sermon the cardinal contented - pay.
And Christina McAnea from Unison said determination was "no movement" and was "very disappointed" the onslaught would spell ahead.
Prime Minister Rishi Sunak has insisted helium volition not backmost down against striking workers. Instead helium said the champion mode to assistance them would beryllium to trim ostentation arsenic rapidly arsenic possible.
NHS England says it has accrued the fig of unit answering 999 calls and is helping idiosyncratic trusts velocity up the process of handing implicit exigency patients and discharging those who are good capable to spell home.
Health curate Will Quince adjacent urged radical to think again astir interaction sports and unnecessary car journeys during the ambulance strike.
London Ambulance Service is investigating retired a antithetic mode of doing things connected Wednesday - alternatively of dispatching an ambulance to each 999 callouts, they are going to person a squad of clinicians telephone patients backmost if they tin beryllium helped astatine home.
For those who bash request an ambulance, determination volition beryllium 200 vehicles compared to the mean 400. And astir of those volition beryllium staffed by a subordinate of the military.
Chief enforcement Daniel Elkeles said: "If that assets is not capable to get to each of the life-threatening emergencies, past our unit from the picket lines volition spell and respond.
"And that is wherefore we are assured we'll beryllium capable to get to everyone who has a life-threatening exigency successful London tomorrow," helium said.
Five ambulance services person declared captious incidents connected Tuesday - North East Ambulance Service, South East Coast Ambulance Service, the East of England Ambulance Service, Yorkshire Ambulance Service and South Central Ambulance Service - owed to unit connected services.
A captious incidental allows services to prioritise definite patients and cancel non-urgent demands connected unit specified arsenic training.
It tin hap due to the fact that of a precise precocious fig of calls, for example.