The Health Secretary has urged the public to "use their common sense" as thousands of ambulance workers and paramedics go on strike in a bitter dispute over pay and conditions.
Steve Barclay, who has said he will not move on the issue of pay, said the emergency ambulance system "will be under very severe pressure today".
He told Sky News: "We're saying to the public to exercise their common sense in terms of what activities they do, being mindful of those pressures that are on the system."
It comes as the Royal College of Emergency Medicine (RCEM) said A&E departments are expecting some patients to turn up to hospital via cars rather than ambulances, and the medical director of NHS England urged people not to get "blind drunk".
Dial 999 'As Usual'
Professor Sir Stephen Powis said ambulance strikes will create a "very difficult day" for the health service, but suggested heart attacks, and strokes would be covered.
He told BBC Radio 4's Today programme: "Today is obviously going to be a very difficult day with the health service.
"But we've been working very closely with the unions to ensure that emergency services for life-threatening conditions are maintained, and that will include stroke and heart attacks.
"There are increased clinicians in call centres to ensure that the right response goes out to the right incident."
Ambulance responses are split into categories, with category 1 being the most life-threatening such as cardiac arrest, while category 2 covers conditions such as stroke, heart attack, and sepsis.
Sir Stephen said strokes fall into the higher end of category 2 cases, and clinicians will determine what response is needed.
He advised people to dial 999 "as usual" if they have a life-threatening condition.
Eight ambulance trusts are currently on their highest level of alert. They have declared critical incidents, which means they cannot provide usual critical services, and patients may be harmed.
Mr Barclay said the Unite, Unison and GMB unions had "refused" to work with the Government at the national level to set out plans for dealing with the strikes.
Speaking on BBC Breakfast, he said unions would only agree local arrangements for life-threatening and emergency calls, which had led to "further uncertainty".
Mr Barclay said trade unions "haven't been willing to work with us to agree national exemptions in terms of covering all of the category 1, category 2, life-threatening, and emergency calls.
"That makes it very difficult for NHS colleagues to plan the contingency measures – working with the military, working with community services, working with first responders – in terms of how we have contingency measures alongside the strikes."
Unite general secretary, Sharon Graham, said category 1 incidents will be responded to immediately and category 2 incidents would be triaged for those that are life-threatening.
She said blame for the strike "lies squarely at the floor of the Government… I've never seen such an abdication of leadership like it in 25 years of negotiating."
'750 Armed Forces Staff'
From a picket line in Longford, Coventry, she also told the PA news agency the Government should get back round the negotiating table "so we can do a deal".
She added: "People are leaving the NHS to go and work in Tesco and Amazon. There is something drastically wrong with that.
"They (ambulance workers) went out when COVID was rife – nobody knew there was going to be a vaccine.
"Now that crisis is over, those exact same workers have been treated like dirt."
Around 750 armed forces staff have been brought in to help ambulance trusts on Wednesday, though they will have a limited role.
They will not be providing clinical care and cannot drive through red lights or turn on blue lights to respond to emergencies.
Mr Barclay, who has accused trade unions of making a "conscious decision" to "inflict harm" on patients, said there was a need to "look forward" to next year's pay process after he declined to review the current offer.
He told the BBC Radio 4 Today programme: "We're already three quarters of the way through this year so what you'd be saying is, go all the way back retrospectively to April to unpick what has been an independent decision by the pay review body.
"But we're already now under way in terms of next year's pay review process, the remit letters have gone out.
"Obviously that body will then consider the changes in inflation, the other issues that have been raised, all as part of the normal process of looking at next year's pay, so we should look forward."
Around 39,000 Patients Suffered with Ambulance Delays Last Month
Rachel Harrison, the GMB union national secretary, accused Mr Barclay of “insulting” ambulance workers with his comments.
Asked if there would be harm to patients as a result of the walkout, she told the BBC: "The sad reality is that patients are being harmed every single day, and that's when we're not on strike.
"The Association of Ambulance Chief Executives themselves report that the increasing number of handover delays and waiting times is leading to harm to patients and deaths.
"So, that's happening when we're not on strike. It's issues like that which have forced our members into this position."
Data analysed by the PA news agency shows 39,000 patients are likely to have experienced potential harm last month as a result of ambulance delays of more than 60 minutes.
Ambulances in England have a target response time of seven minutes for category 1 calls but this has not been met nationwide since April 2021.
In November, the average response time for these incidents outside London was 9 minutes and 26 seconds. Data for London is not currently available.
Meanwhile, the target response time for category 2 calls is 18 minutes.
This has not been met nationwide since July 2020, while last month the average outside London was 48 minutes and eight seconds.
Speaking earlier, Sir Stephen told BBC Radio 4's Today programme that the public needed to be "sensible" during a "difficult day".
He said: "It's the season of parties, pre-Christmas, so do enjoy yourself but obviously don't get so drunk that you end up with an unnecessary visit to A&E.
"That's good advice at the best of times and certainly on today when we know that services are stretched.
"Certainly today is not the best of days to end up being in an A&E department if you don't need to be there, if you've got yourself blind drunk that doesn’t sound like fun to me."
Meanwhile, Matthew Taylor, chief executive of the NHS Confederation, which represents NHS organisations, wrote to Prime Minister Rishi Sunak on Tuesday saying NHS leaders could not keep patients safe and urged him to negotiate on pay.
Speaking on BBC Breakfast on Wednesday, he said "we cannot afford to drift into a winter of industrial action" as he urged the Government and unions to reach an agreement.
Elsewhere, Dr Adrian Boyle, president of the Royal College of Emergency Medicine, said the emergency system had been under "immense pressure" for the last three years, telling Times Radio: "Trying to work out the effect of industrial action compared to a system which is already not doing what we want it to do is going to be difficult."
He added: "We're expecting people with strokes and heart attacks to turn up at the front door (on Wednesday). Now, because of the delays this has already been happening quite a lot anyway."
This article contains information from PA Media.