The opening of the the new £150m critical care unit at the Royal Victoria Hospital in Belfast is to be delayed further because of a problem with the pipework.
Corrosion has been found in the building's new heating system.
It means a major repair job has to be completed before it can open to patients.
The delay will be longer than first thought, as workers are employed trying to fix the plumbing problem.
It is understood the damage to the pipes was detected in the hot water heating system during final checks.
The main emergency department for Belfast, and intensive care for patients across Northern Ireland, will fill the first six floors. The top three floors will eventually house maternity services.
The new 12-storey building was due to be handed over to the Belfast Health Trust in November 2012, but that deadline was missed.
The Department of Health has confirmed the trust will not take control of the building until 2014 at the earliest.
"Significant delays in the completion of this project have been due to problems with corrosion in the closed water systems identified in the commissioning process," it said.
"Handover of the completed fully commissioned building is now expected in 2014."
Once the trust does take ownership of the new state-of-the-art building, it is thought it will be another three months before it is ready to admit patients.
UNISON health trade union spokesman Ray Rafferty, who is based at the Royal, said his main concern was "that the building is going to be delayed by 12 to 14 months".
"That means services are delayed, and patients are going to suffer because of these delays," he added.
"The knock-on effects are many, we have an emergency department which is in a temporary building. The building is not fit for purpose."
The main contractor McLaughlin and Harvey declined to discuss the cost of the repair work, answering "no comment" to any queries, but a letter from the Health Minister Edwin Poots, seen by the BBC, said the cost would be "significant".
UNISON estimates the repair work could cost anywhere between £2m and £5m.
The Department of Health said the main contractor was responsible for the repairs.
A spokesperson said: "There are no additional costs for the department from this corrosion problem."
The correspondence from the health minister said: "It may be necessary to incur expenditure like additional design fees, expert opinion etc which would then have to be recovered through settlement of the case, whether through the application of contract conditions, or if necessary, litigation."
The minister added: "The main contractor McLaughlin and Harvey, and the mechanical sub-contractor, Vaughan Engineering, have both advised their insurance companies of the issue as the cost of rectification will be significant."