Nurses in England, Wales and Northern Ireland have started a nationwide strike in the largest action of its kind in NHS history.
Staff will continue to provide "life-preserving" and some urgent care but routine surgery and other planned treatment is likely to be disrupted.
The Royal College of Nursing said staff had been given no choice after ministers refused to reopen pay talks.
RCN general secretary Pat Cullen has called on the government to "do the decent thing" and resolve the dispute before the year ends.
Ms Cullen told BBC Breakfast the strike marked "a tragic day in nursing".
"We need to stand up for our health service, we need to find a way of addressing those over seven million people that are sitting on waiting lists, and how are we going to do that? By making sure we have got the nurses to look after our patients, not with 50,000 vacant posts, and with it increasing day by day," she said.
Health Minister Maria Caulfield, a former nurse, accepted "it is difficult" living on a nurse's wage, but said that a 19% pay rise "is an unrealistic ask".
Under trade union laws, the RCN has to ensure life-preserving care continues during the 12-hour strike.
Chemotherapy and kidney dialysis should run as normal, along with intensive and critical care, children's accident and emergency and hospital neonatal units, which look after newborn babies.
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Source: BBC News, 15 December 2022