Patients are facing a week of disruption, with more than 10,000 outpatient appointments and surgeries cancelled in Belfast.
Some people referred by their GPs on suspicion of cancer could have their diagnosis delayed, the head of the Belfast Trust has said. The trust apologised, blaming industrial action on pay and staffing.
Martin Dillon said outpatient cancellations "could potentially lead to a delay in treatment" for cancer.
The Department of Health said the serious disruption to services was "extremely distressing".
Source: BBC News, 2 Decmeber 2019