MPs have expressed fears a contingency plan could lead to the eventual closure of emergency services at Daisy Hill Hospital in Newry, County Down.
The £1m contingency plan has been approved for Craigavon Area Hospital, in case Daisy Hill's emergency services have to be suspended at night.
The trust had told staff at the Newry hospital that overnight closure may be unavoidable due to staff shortages.
MPs for Newry and Armagh and South Down said 24-hour service must be retained.
Margaret Ritchie told the BBC that in the event of a medical emergency, patients living in parts of her South Down constituency would be too far away from Craigavon.
"If there's no emergency services at Daisy Hill at the weekend or at night, there will be a constituency - and people in inaccessible places like the Mournes - having to travel nearly two hours to get to Craigavon, in grave levels of distress and pain," she said.
"That is totally unacceptable."
Newry and Armagh MP, Mickey Brady, said he believed the health trust was "set on a course of closing Daisy Hill emergency department".
'Alarm and concern'
The Sinn Féin representative was among politicians who attended Thursday's meeting at the Southern Health Trust's headquarters in Craigavon.
"I had hoped that the trust would have been offering a solution to the ongoing concerns about staff cover at Daisy Hill Emergency Department, however, what was announced was a recipe to ensure its eventual closure and ultimately the removal of its acute hospital status," he said.
"We cannot allow this to happen."
Ms Ritchie said the issue was a "matter of grave disquiet, alarm and concern" for her constituents and urged the board not to "betray" patients and staff at Daisy Hill.
"Ultimately this investment will expand Craigavon at the expense of Daisy Hill Hospital," the SDLP MP said.
'Unable to cope'
However, Mr Brady said that Craigavon was already struggling to deal with its own patients.
"I am aware that Craigavon Hospital has redirected ambulances away from its emergency department on numerous occasions in recent months as it was unable to cope with the volume of patients.
"There is no logic to the trust considering a reduction of service in Daisy Hill when Craigavon is already over-stretched."
In a statement on Thursday, the trust said that contingency plans were being considered to ensure patients can continue to access safe, high-quality care in the unfortunate event that medical cover cannot be secure for more than one night.
It was understood that while there should be about 10 senior permanent doctors employed, at the moment the trust has just two permanent senior medics.
One of these is part-time and the remaining shifts are being filled by locums.
The trust said attempts to recruit have been unsuccessful, and it asked the Southern Trust Board to approve preparations in Craigavon Area Hospital to accommodate an additional flow of patients should the Daisy Hill service be suspended at night.