One of the A&Es expected to treat up to 12,000 extra emergency patients every year following the closure of the Belfast City Hospital casualty unit is already struggling to cope, it has emerged.
Latest government figures have revealed the Ulster Hospital A&E had the highest number of patients waiting longer than 12 hours in Northern Ireland - 617 between July and September this year.
The figures have been released as bosses at the South Eastern Health & Social Care Trust warned patients will probably face longer waits for treatment and beds as a result of the closure of the City Hospital A&E at 8am on Tuesday.
They have also revealed the number of pre-planned surgical procedures carried out at the hospital each week will be halved in November from 60 to 30 to ensure there are enough beds to cope with additional emergency cases.
"It is going to be challenging but I would ask for understanding and patience from the public as our staff are working as hard as they can," said Seamus McGoran, director of hospital services at the Trust.
According to the statistics, the Royal Victoria Hospital's A&E has also struggled to cope with the number of patients turning up - 95 people waited in the unit longer than 12 hours last month.
The casualty departments at the Royal Victoria, Mater and Ulster hospitals are expected to deal with the 40,000 people who visit the City Hospital A&E every year.
The Belfast Health & Social Care Trust has said it has to close one of its A&Es because of a shortage of consultants.
Colm Donaghy, chief executive of the Trust, said they have been forced to reduce the number of A&Es to allow them to concentrate staff on two sites.
He said this will improve supervision of junior doctors, patient safety and will mean senior clinicians will be on hand to make decisions more frequently.
However, there are serious concerns that the remaining A&Es in and around Belfast will struggle to cope with additional patient numbers.
Mr McGoran said: "We would have liked more time to prepare for these changes but that hasn't been possible.
"We have been working closely with colleagues in the Belfast Trust and Northern Ireland Ambulance Service to ensure that we meet the needs of our patients during this time.
"We are putting in two additional 20-bed medical wards but these won't be ready for Tuesday.
"The first ward will not open until the second week of November and the second ward will open in the first week in December.
"We are recruiting additional staff, including consultants, nurses and domestic staff, and the Health & Social Care Board has given a commitment of £3.5m to support additional capacity.
"We will also reduce the number of elective procedures during November to ensure there are enough beds in the hospital."
Number of people who waited more than 12 hours in A&Es from July 1, 2011 until September 30, 2011.
Antrim Area: 299
Craigavon Area: 0
Belfast City: 29
Royal Victoria: 146
Royal children's hospital: 0