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 strike 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