The emergency department at Newry's Daisy Hill Hospital is "untenable" because of a chronic shortage of consultants, a union said.
The centre is eight staff down and using locum doctors as cover, NIPSA representative Brian Smyth added.
Around 52,000 patients attended in the past year.
Mr Smyth said: "At the moment, the actual number (of consultants) in post is 1.8 and the service is only being maintained by using locums to fill the gaps.
The (Southern Health and Social Care) Trust say that this is untenable, especially with the summer holidays approaching."
Mr Smyth said the number of consultants required was 10.
The trust has informed NIPSA that it has attempted on several occasions to recruit to these posts with no success.
The trust also indicated that it will do everything in its power to keep the emergency department running on a 24-hour basis.
Tommy Brownlee, another NIPSA representative, said: "The trust's statement will be cold comfort to the 52,000 patients who attended the emergency department in the last year across its catchment area and for our members who run the service.
"The trust has indeed tried to fill the posts, but this all points to a failure in the long-term planning of the health service.
"NIPSA believe this is a vital service and must be retained."
A statement from the trust said, despite numerous recruitment efforts over the past two years, nationally and internationally, there continued to be a shortage of permanent senior medical staff.
It said the service was supported mainly by locum medical staff and the continuing goodwill of clinical staff across the trust.
The statement said: "The service currently remains vulnerable to the short-notice withdrawal of senior medical cover, particularly during the summer months when there is a less secure supply of locum consultants.
"Therefore, if there is any risk to patient safety due to lack of senior medical cover at night, the emergency department will temporarily suspend the service until suitable medical cover becomes available.
"This would be a last resort, and the trust is making every effort to avoid any reduction in hours in the short-term."