Several life-saving kidney transplants within the Belfast Health Trust area could not go ahead at the weekend because not enough nurses were available to support surgeons.
The trust was offered healthy kidneys from deceased donors.
But they were turned down as the operations could not be carried out without sufficient staff.
The trust has apologised and blamed the staff shortage on a rise in the number of Covid-19 patients in hospitals.
The cancellation of the transplants comes after some health trusts cancelled cancer surgeries at the end of July.
On Monday the Royal College of Surgeons said those cancellations could have been avoided.
It told BBC News NI that proposals it had made to stop surgeries being cancelled amid an increase in the number of Covid-19 patients had not been implemented.
‘Last-resort decision’
The Belfast Health Trust said it had come under “significant pressure” as more people with Covid-19 have been admitted to intensive care units.
It said it therefore had to make the “difficult decision” to cancel some kidney transplants.
That, it said, allowed staff who work in operating theatres to “assist in intensive care units at this challenging time”.
All options were explored in order to try and ensure transplants could be carried out – this decision was a last resort and “not taken lightly,” said the trust.
“We apologise to all patients and we are doing all we can to ensure we can reschedule surgeries as soon as possible.”
Last year a record number of kidney transplants were carried out in Northern Ireland in spite of the coronavirus pandemic.
But in November the transplant system was suspended because of the Covid-19-related pressure on the health service.
In January BBC News NI reported that organs were regularly turned down instead of being used for transplants because no operating facilities were available.
The issue was raised in the House of Commons by the Democratic Unionist Party (DUP) MP Ian Paisley, who warned against a “scandal of discarding viable transplant organs from developing”.