An estimated 100,000 nurses are striking at 76 hospitals and health centres, cancelling an estimated 70,000 appointments, procedures and surgeries in Britain's state-funded NHS.
Britain is facing a wave of industrial action this winter, with strikes crippling the rail network and postal service, and airports bracing for disruption over Christmas.
Inflation running at more than 10%, trailed by pay offers of around 4%, is stoking tensions between unions and employers.
Of all the strikes though, it will be the sight of nurses on picket lines that will be the stand-out image for many Britons this winter.
"What a tragic day. This is a tragic day for nursing, it is a tragic day for patients, patients in hospitals like this, and it is a tragic day for people of this society and for our NHS," Pat Cullen, the head of the Royal College of Nursing (RCN) union, told the BBC on a picket line.
The widely admired nursing profession shut down parts of the NHS, which since its founding in 1948 has developed national treasure status for being free at the point of use, hitting healthcare provision when it is already stretched in winter and with backlogs at record levels due to COVID delays.
Health minister Steve Barclay said it was deeply regrettable that the strike was going ahead.
"I’ve been working across government and with medics outside the public sector to ensure safe staffing levels - but I do remain concerned about the risk that strikes pose to patients," he said.
The industrial action by nurses on Dec. 15 and Dec. 20 is unprecedented in the British nursing union's 106-year history, but the RCN says it has no choice as workers struggle to make ends meet.
Nurses want a pay rise of 5% plus inflation, arguing they have suffered a decade of real-terms cuts and that low pay means staff shortages and unsafe care for patients. The government says their demand would equate to a 19% hike.
The government has refused to discuss pay, which Cullen said raised the prospect of more strikes into next year.
"Every room I go into with the secretary of state, he tells me he can talk about anything but pay," she said. "What it is going to do is continue with days like this."
Barclay told reporters: "I do think it's important that we have a constructive engagement but it's got to reasonable."
Outside St Thomas' Hospital in central London, Ethnea Vaughan, 50, a practice development nurse said she felt nurses had no option but to strike, blaming a government that had ignored their concerns for years.
"Nothing is changing and I've been in nursing for 27 years and all I can see is a steady decline in morale," she told Reuters.
In Belfast, passing vehicles sounded their horns in support of the nurses gathered on picket lines in below freezing temperatures outside the Royal Victoria Hospital.
"I didn't make this decision lightly ... I decided it was time to say 'enough'," said Louise Mitchell, who has been a nurse for 40 years.
"We don’t want patients to suffer any more. Patient care is suffering every day of every week in this country because there is not enough resources in the health service."
The government in Scotland avoided a nursing strike by holding talks on pay, an outcome that the RCN had hoped for in England, Wales and Northern Ireland.
But the government has said it cannot afford to pay more than the 4-5% offered to nurses, which was recommended by an independent body, and that further pay increases would mean taking money away from frontline services.
Some treatment areas were exempt from the strike, the RCN has said, including chemotherapy, dialysis and intensive care.
Polling ahead of the nursing strike suggested a majority of Britons supported the action.