The number of people phoning 999 appears to have dropped in some parts of England as thousands of ambulance staff and paramedics strike until midnight.
Some ambulance trusts reported receiving fewer calls, with the West Midlands Ambulance Trust thanking people for heeding their advice to only call in an emergency.
But health leaders urged people to still call for an ambulance if they were experiencing a life-threatening emergency, amid fears that even those who needed help would not call.
Dr Adrian Boyle, president of the Royal College of Emergency Medicine (RCEM), told the PA news agency: “There may be a number of reasons why 999 calls are dropping – hesitancy may be a key factor during the industrial action.
“We want to reassure patients and the public that if they need emergency care, A&Es remain open.”
A West Midlands Ambulance Service spokesman told PA it had been experiencing fewer calls than normal.
There, the GMB and Unite unions have agreed to respond to category 1 and serious category 2 calls.
A spokesman said: “We are seeing a reduction in calls today and are grateful to the public for heeding the advice we issued yesterday.”
It said category 2 calls requiring a response were included in what union staff had agreed to cover.
The Welsh Ambulance Service told PA “demand is manageable”.
In a statement it added: “Demand continues to be managed, but any influx of calls would put significant pressure on our service, so please only call 999 in a life-threatening emergency.”
Meanwhile, the East Midlands Ambulance Service said it was too early to say how the service was coping.
Its advice to the public remained to only call 999 if there was a risk to life or if somebody was seriously ill or injured.
Up to half of its more than 4,000 workforce were GMB members who were striking.
The trust said it had agreed exemptions with the union for staff to either attend category 1 calls or category 1 calls and the most serious category 2 calls such as chest pain, strokes, gynaecology emergencies where mother or baby are at risk, road traffic collisions where a patient is trapped, and unwell children aged five and under.
South Central Ambulance Service told PA its main impact from strikes was patient transport services in Sussex and Surrey, rather than urgent and emergency care services.
The London Ambulance Service declined to comment on how services were running.
The BBC reported that Unison officials had said demand for ambulances in south-west England was down by a third.
Meanwhile, a chief executive of a large northern teaching trust told the Health Service Journal (HSJ) it had “so far not (been) as bad as I’d feared in terms of hospital pressures – in fact, (emergency departments) are less pressured than usual.
“We haven’t seen cars/taxis with patients arriving in large numbers but the problem is that much of the risk is not currently visible to us given people will be at home …
“We therefore expect very busy days on Thursday and Friday.”
It comes as the Royal Albert Edward Infirmary in Wigan said it was full after facing “unprecedented pressures” in A&E.
The Wrightington, Wigan and Leigh NHS Foundation Trust tweeted: “Due to the immense pressure on our services, the trust has declared a critical incident at Royal Albert Edward Infirmary in Wigan.
“Our A&E is full. Do not attend, unless you have a life or limb-threatening emergency.”
Earlier on Wednesday, there was a bitter war of words between unions and Health Secretary Steve Barclay, who has said he will not back down on pay.
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. But the unions said all those agreements had been made locally and were in place.
Unite general secretary Sharon Graham also accused Mr Barclay of a “blatant lie” for saying ambulance unions had taken a “conscious decision” to inflict harm on patients.
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.”
Earlier, Mr Barclay 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 that “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.”
A paramedic based in Nottinghamshire told PA that patients’ lives have been at risk for a long time due to issues in the NHS.
Tom, 33, from the East Midlands Ambulance Service, said: “I’ve attended elderly patients who have been on the floor with broken hips for over 20 hours.
“They’ve been waiting that long that their limbs have started to become necrotic (dying tissue), resulting in major surgery to remove said limbs.”
He said at one point 11 ambulances were stuck at hospital waiting to hand over patients to A&E.
“The conditions we work in on a regular basis don’t enable us to do the job we want to do to its full capacity and is putting patients’ lives at risk long before strikes were even considered,” he said.
Meanwhile, the medical director of NHS England has urged people not to get “blind drunk”.
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.”
Eight ambulance trusts are on their highest level of alert. They have declared critical incidents, which means they cannot provide usual critical services and patients may be harmed.
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