"Significant" IT issues at services across the Southern Health Trust will continue into Thursday with the majority of planned surgery and out-patient hospital appointments being cancelled.
A major incident is still ongoing at sites, including Craigavon and Daisy Hill hospitals, as ambulances are being diverted while work continues to resolve the problems, the trust said.
Patients were warned not to come to appointments at any hospital or community service in the trust's area on Wednesday.
One patient at Daisy Hill hospital said she is "devastated" that she can not get an operation she has been waiting nearly a year for.
