Patients are facing a week of disruption, with more than 10,000 outpatient appointments and surgeries cancelled in Belfast.
The Belfast Trust has apologised, saying it was due to ongoing industrial action by unions over pay and staffing.
But the head of the Royal College Nursing said it was “devastated” by the decision to cancel appointments.
The Department of Health said the serious disruption to services was “extremely distressing”.
RCN director Pat Cullen said her union’s members were “absolutely devastated” at such widespread cancellations.
“We, as yet, have no idea why such significant appointments will be cancelled because our nurses will be there to care for their patients,” she told BBC NI’s Good Morning Ulster programme.
Ms Cullen said nurses planned to take industrial action on Tuesday, but would continue to do all direct patient care.
They will not pick up on the tasks usually performed by porters or administration.
“We have been describing what has been a perfect storm in terms of the levels of vacancies within the nursing workforce and that has fallen on deaf ears,” she said.
“Nurses cannot continue to work in the environment they are working in.”
The trust is asking patients not to phone to re-schedule due to the large numbers of people affected.
The South Eastern Health and Social Care Trust is also cancelling some services but on a smaller scale.
Last week, the Royal College of Surgeons warned Northern Ireland’s healthcare system was “at the point of collapse” after waiting times reached an all-time high.
Where are the cancellations?
In Belfast, the following hospitals will have all outpatient appointments, day case procedures and planned surgeries cancelled on Monday, Tuesday and Thursday:
- Royal Victoria Hospital (including the children’s hospital, maternity hospital and school of dentistry)
- Belfast City Hospital
- Mater Hospital
- Musgrave Park Hospital
However, some outpatient appointments will go ahead at these places:
- Health and wellbeing centres
- Community facilities
- Chemotherapy or radiotherapy at either the Bridgewater suite or cancer centre at Belfast City Hospital
- Macular clinic at Fairview
- Paediatric oncology at the Children’s Hospital
- Obstetrics or antenatal appointment at Royal Jubilee Maternity Hospital
On Wednesday, all outpatient services will run as normal. Planned surgeries and day case procedures will still be postponed.
Emergency services and day centres are not affected by the industrial action.
In the South Eastern area, all routine outpatient appointments at the Ulster Hospital have been cancelled on Wednesday, with the exception of maternity and children’s services.
On Friday, outpatient departments at Ulster Hospital, Lagan Valley Hospital, Downe Hospital, Ards Hospital and Bangor Hospital will be closed in the afternoon.
Full details and advice can be found on the Health and Social Care Board website.
Health workers across Northern Ireland are staging industrial action in protest at pay and staffing levels which they claim are “unsafe”.
The Department of Health said its offer of an independent conciliation process on the dispute stands.
“As we have been saying for some time, we simply don’t have the money or authority to resolve this impasse, and we strongly believe that patients should not be the ones to suffer because of that,” the department said.
“Like our Trust colleagues we would still wish to apologise to everyone affected.”