The new critical care building at Belfast's Royal Victoria Hospital will open eight years late, the BBC understands.
The intensive care unit (ICU) will not become fully operational until at least autumn 2020.
A consultant has described the development as "extremely disappointing".
Dr Brian McCluskey, who was involved in the original design, said patient safety must come first.
"We are all disappointed, not so much for ourselves but for our patients, because... whilst our patients are being very well cared for in the existing ICU... it would be nice if they were getting that additional privacy and dignity," he said
"But we know that the ICU will be open and it will be open for a very long time."
Dogged by problems
The 12-storey building in the grounds of the Royal Victoria Hospital has been dogged by problems.
Due to open in 2012, the state-of-the art £150m building houses the Emergency Department, which opened its doors in 2015 due to winter pressures.
BBC News NI understands that millions of additional pounds have had to be spent correcting flaws including ripping out equipment that has become out-of-date due to the ongoing delay.
- The ventilation, sewage and drainage systems have all been the source of problems
- Operating theatres have had to be remodelled and brought up to standard
- The helipad remains non-operational
Among the questions being asked is how much, if any, the delay is costing the public purse.
A spokesperson for the Department for Health said the Belfast Trust continues to update it on the delays.
"Whilst remedial and additional works to the facility have been necessary, costs to date remain within the approved investment for the project," the spokesperson added.
"As with all capital projects, the trust will be required to conduct a post-project evaluation which, in this case, will include a review of the events that took place and any lessons learned."
Despite both the size and demand for this hospital and its services, many people seem to have forgotten about it.
Numerous politicians had to be reminded about it when contacted by the BBC - and it is fair to say that both MLAs and medical unions had totally forgotten that the hospital even existed when contacted by the BBC for comment.
While the additional costs could reach up to £10m, the BBC understands that an agreed out-of-court settlement with contractors may help provide some additional funding.
The current delay is over ventilation work being carried out on theatres in the ICU at an additional cost of over £3m.
The deputy director of the Royal College of Nursing, Garrett Martin, said the ongoing delay was "incomprehensible and totally unacceptable".
Mr Martin said the intensive care unit was designed to treat the sickest of patients and the fact it would not be operational for over another year was difficult to understand.
The original contractors for the building were McLaughlin and Harvey.
In 2017, when the BBC contacted them with a series of questions, they said they had no comment.
At the time, it was also reported that an internal report by the new contractors, Killowen Contracts Ltd and Michael Nugent Ltd, highlighted that new problems were being discovered on a "regular basis".
The BBC has contacted McLaughlin and Harvey for comment.