Nurses in England, Wales, and Northern Ireland are to go on strike from 08:00 on Thursday 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 are likely to be disrupted.
The Royal College of Nursing said staff had been given no choice after ministers refused to reopen pay talks.
The UK government said the RCN’s 19% pay rise demand was unaffordable.
The action will involve nurses in around a quarter of hospitals and community teams in England, all health boards in Northern Ireland, and all but one in Wales. Nurses are not striking in Scotland.
Under trade union laws, the RCN has to ensure life-preserving care continues during the 12-hour strike.
Beyond that, it will be up to NHS boards and trusts to negotiate services on a local level, with discussions likely to come down to the circumstances of some individual patients.
The biggest impact is likely to be in pre-booked treatment such as hernia repair, hip replacements, or outpatient clinics.
The NHS has said it is “vital” people continue to come forward for emergency care during the strikes and anyone not contacted to reschedule an appointment should attend as planned.