About 9,000 nurses in Northern Ireland are to launch 12 hours of strike action at 08:00 GMT on Wednesday morning.
They belong to the Royal College of Nursing (RCN), which has not participated in strike action in its 103-year history.
Alongside the other health unions, they are protesting over pay and patient safety.
Other nurses, who are members of Unison, will also be walking out.
Last night leaders of the five main political parties in Northern Ireland met the head of the NI Civil Service David Sterling and health permanent secretary Richard Pengelly in a bid to avert the strike action.
‘Much reduced service’
The letter to Julian Smith signed by the five party leaders said there was “collective support for the restoration of pay parity”.
Following their meeting with the Civil Service, the parties wanted to meet the secretary of state but the Northern Ireland Office said health remained a devolved matter.
Dr Tony Stevens, chief executive of the Northern Health Trust, told BBC News NI that while the health trusts will be delivering a “much reduced” service, they will be able to deliver services safely.
“Speaking on behalf of all of the trust chief executives, we along with the trade unions and staff have been working really hard in the last three days,” he said.
“We are satisfied that we will be delivering a much reduced service, but we will be able to deliver critical services safely during the next 24 hours.”
2,800 unfilled nursing posts
According to the RCN, nurses’ pay within the health service continues to fall behind England, Scotland and Wales.
It argues that the real value of nurses’ pay here has fallen by 15% over the past eight years.
There are just under 2,800 unfilled nursing posts within the health service in Northern Ireland.
The RCN estimates that a similar level of unfilled posts exists within nursing homes.
The nursing vacancy rate in Northern Ireland is 13%, compared to about 11% in England and 6% in Scotland.
This means that for every eight nurses who should be working in Northern Ireland, one is missing.
Nurses are demanding better workforce planning.
Last year, the local health service spent £52m on agency nurses to fill these gaps in the workforce.
That money, the RCN argues, could be better managed to train and pay health service nurses.
Talks between management are continuing to ensure critical departments including cancer services and emergency departments are covered.
A spokesperson for Health and Social Care Board (HSCB) said major challenges were expected across all health and social care services on Wednesday.
It also advised that if patients or service users have not been contacted about their Trust then they should attend their appointment/ service as normal.
All emergency departments remain open, but “significant pressure” was expected within the departments.
“The priority will be on the treating emergency and life threatening conditions first. Patients with less urgent conditions may have to wait for lengthy periods,” said the spokesperson.
Pat Cullen from the RCN said: “No time is a good time to strike, but years of negotiations between various health ministers failed and years of warnings were ignored.”
The RCN plans to stage picket lines in hospitals across Northern Ireland.
The heads of all of Northern Ireland’s health trusts have stated the current crisis in the service has been “years in the making”.