A review of services at the Royal Victoria Hospital ordered by Health Minister Edwin Poots is a clear indication it is in crisis, health unions have said.
The minister has confirmed the urgent review will be carried out by external experts from watchdog body, the Regulation and Quality Improvement Authority.
The move has been welcomed by both unions and politicians but they have also warned the review must be "thorough and listen to staff on the frontline".
The reassessment follows a major incident in the hospital's accident and emergency department last month, when it became necessary to call in more staff to help clear a backlog of patients.
Following the incident on January 9, Mr Poots rejected claims that the A&E service was in crisis.
But yesterday he said: "I am seeking assurance for the public and for myself as minister that our EDs are providing care of the highest quality and safety."
Mr Poots said he will receive a report on the findings within the next few weeks.
Janice Smyth, director of the Royal College of Nursing in Northern Ireland said the experts' investigation must be a "truly independent" review of services.
Ray Rafferty from Unison, the union representing hospital workers, said: "We would demand a full independent investigation is carried out."
Sinn Fein chairwoman of the health committee Maeve McLaughlin added: "I assume the minister is recognising that there is a crisis in emergency care and that he has finally decided to act."
The SDLP's health spokesperson Fearghal McKinney insisted the review must not be "a kick into the long grass".
In a statement from the Belfast Health and Social Care Trust they said: "We welcome any opportunity to learn and look forward to assisting the expert review team."