Major incident declared on Saturday by Northern Health Trust
Northern Health Trust declared a major incident on Saturday night, BBC News NI understands.
A major incident means there are too many patients for staff to deal with safely. 
Senior trust executives took the decision after the emergency department at Antrim Area Hospital reached full capacity. 
On Sunday the Belfast Trust appealed to any available nursing staff for help during the nightshift.
rust said it was "under extreme pressure" due to an increasing number of sick patients.
Northern Trust used social media to appeal to the public "not to attend under any circumstances. "
In a statement on Sunday, the trust said it had warned for some time that such a situation could happen.
A spokesperson said a "disproportionately high number" of critically ill patients arrived at Antrim Area Hospital in quick succession, leading to the senior emergency department consultant that the department could no longer operate safely.
"This was a regrettable position to be in, but in the circumstances it was the only safe and appropriate response," they added.
"The trust is continuing to work closely with the Northern Ireland Ambulance Service and other Trusts to coordinate an appropriate response given regional pressures."
Declaring a major incident is rare and a sign the hospital is under extreme pressure.  
Under such circumstances there is a call out for staff to come into work. 
Ambulances were also redirected to use other hospitals. 
A potential major incident was declared at Antrim Area Hospital in February but was stood down after several hours.
On Sunday morning the trust said the emergency department remained under extreme pressure and asked people only to attend if their condition was life-threatening or urgent.
It stressed that people in such circumstances should attend, but anyone else should stay away.
"The Antrim site currently has a high number of very ill patients who require admission into the hospital and therefore our priority today has to be patient discharge," the trust spokesperson said.
Alliance Party assembly member for East Antrim, Danny Donnelly, said urgent action was needed to prevent a major incident happening again.