Patients are facing a week of disruption, with more than 10,000 outpatient appointments and surgeries cancelled in Belfast.
Some people referred by their GPs on suspicion of cancer could have their diagnosis delayed, the head of the Belfast Trust has said.
The trust apologised, blaming industrial action on pay and staffing.
Martin Dillon said outpatient cancellations “could potentially lead to a delay in treatment” for cancer.
The Department of Health said the serious disruption to services was “extremely distressing”.
Speaking on Radio Ulster’s The Nolan Show, Mr Dillon, the Belfast Trust chief executive, said: “There are some patients whose GPs may well have a suspicion of cancer – not yet confirmed.
“Some of those people will have been due to have tests this week that would have confirmed that one way or the other. Regrettably, we’ve had to stand those appointments down.
“This could potentially lead to a delay in diagnosis and could potentially lead to a delay in treatment.”
Mr Dillon confirmed that all patients who are undergoing active cancer treatment will have their treatment as planned this week.
Members of the Unison union are carrying out waves of four hour strikes on Monday.
Speaking on BBC NI’s Good Morning Ulster, Patricia McKeown, regional secretary of Unison, argued that three of the trusts had announced they would cancel all elective surgery some weeks ago, before her union confirmed they were taking industrial action.
“They cited the staffing crisis and it is the staffing crisis and absence of pay that has brought our members to industrial action,” she said.
Ms McKeown said she was sorry that appointments were being cancelled.
“This is not of our making. I wish the people who run the system would put as much energy into every day of the week when they are cancelling surgery and outpatient appointments every day of the week, when there is no industrial action,” she said.
“I wish they would put as much energy into getting the resources back into the system as they are in criticising health workers who have been pushed to the brink.”
She said the industrial action would be “cumulative” and it would be “an escalation”.
“Right across the whole system, Unison members are working to rule.
“They are coming in to do their contracted jobs, they are stopping giving the tens of thousands of free hours they give to the health service every week in order to cover for missing colleagues.”
Direct patient care
Royal College of Nursing director Pat Cullen said her union’s members were “absolutely devastated” at such widespread cancellations.
“We, as yet, have no idea why such significant appointments will be cancelled because our nurses will be there to care for their patients,” she told BBC NI’s Good Morning Ulster programme.
Ms Cullen said nurses planned to take industrial action on Tuesday, but would continue to do all direct patient care.
They will not pick up on the tasks usually performed by porters or administration.
“We have been describing what has been a perfect storm in terms of the levels of vacancies within the nursing workforce and that has fallen on deaf ears,” she said.
“Nurses cannot continue to work in the environment they are working in.”
The trust is asking patients not to phone to re-schedule due to the large numbers of people affected.
The South Eastern Health and Social Care Trust is also cancelling some services but on a smaller scale.
Last week, the Royal College of Surgeons warned Northern Ireland’s healthcare system was “at the point of collapse” after waiting times reached an all-time high.
Where are the cancellations?
In Belfast, the following hospitals will have all outpatient appointments, day case procedures and planned surgeries cancelled on Monday, Tuesday and Thursday:
- Royal Victoria Hospital (including the children’s hospital, maternity hospital and school of dentistry)
- Belfast City Hospital
- Mater Hospital
- Musgrave Park Hospital
However, some outpatient appointments will go ahead at these places:
- Health and wellbeing centres
- Community facilities
- Chemotherapy or radiotherapy at either the Bridgewater suite or cancer centre at Belfast City Hospital
- Macular clinic at Fairview
- Paediatric oncology at the Children’s Hospital
- Obstetrics or antenatal appointment at Royal Jubilee Maternity Hospital
On Wednesday, all outpatient services will run as normal. Planned surgeries and day case procedures will still be postponed.
Emergency services and day centres are not affected by the industrial action.
In the South Eastern area, all routine outpatient appointments at the Ulster Hospital have been cancelled on Wednesday, with the exception of maternity and children’s services.
On Friday, outpatient departments at Ulster Hospital, Lagan Valley Hospital, Downe Hospital, Ards Hospital and Bangor Hospital will be closed in the afternoon.
Full details and advice can be found on the Health and Social Care Board website.
Health workers across Northern Ireland are staging industrial action in protest at pay and staffing levels which they claim are “unsafe”.
The Department of Health said its offer of an independent conciliation process on the dispute stands.
“As we have been saying for some time, we simply don’t have the money or authority to resolve this impasse, and we strongly believe that patients should not be the ones to suffer because of that,” the department said.
“Like our Trust colleagues we would still wish to apologise to everyone affected.”