More than 2,700 people are waiting for an urgent colonoscopy to test for bowel cancer, while almost 19,000 people overall are waiting for a colonoscopy.
Figures up to the end of May, which were published today by the Irish Cancer Society, show the impact that Covid-19 is having on bowel cancer screening services.
It said it has put additional pressure on what it described as under-resourced endoscopy services.
It is calling for urgent action to ensure that people are not left waiting for this cancer test, which they say could lead to a delayed diagnosis.
CEO Averil Power said of the more than 2,700 people waiting for an urgent colonoscopy, more than 1,300 people are waiting longer than the target of 28 days for a test and more than 300 people are waiting for more than 90 days.
"These are people who need a colonoscopy because they have shown certain symptoms. Thousands more are considered non-urgent and are facing unacceptable wait times also," she said.
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Last week, Minister for Health Simon Harris said the Health Service Executive confirmed that there would be restart dates for cancer screening programmes by the end of this month.
Ms Power said it was crucial that people are diagnosed early to make sure they have the best chance of survival and a higher quality of life after cancer.
"We have been concerned for some time about the under-funding and under-resourcing of the endoscopy services in Ireland and now due to the pandemic things have reached a critical point," she said.
Ms Power said it was unrealistic to expect that endoscopy services could catch up with the urgent demand with their current funding, staffing and resources.
She said the Irish Cancer Society was calling on the Government to provide additional resources, including physical space to allow for adherence to public health guidelines, additional staff and extended operational hours where appropriate, as well as the maximisation on available capacity in the public and private systems.
Consultant Gastroenterologist Dr Anthony O'Connor said: "People who require an urgent colonoscopy should be seen in 28 days under best practice.
"This is not happening for too many people and many more people will be left waiting for far too long unless we see steps taken now.
"The longer people have to wait the greater risk we are taking with their outcome if they are found to have cancer.
"The entire health system is playing catch up due to Covid-19 but endoscopy was already behind."
In response the HSE has said its immediate priority is to provide endoscopy for patients who have been waiting for urgent procedures for more than 28 days.
It said that as of 5 June there are 1,250 patients in this group.
The HSE said that on 4 June, endoscopy units were issued with national guidance on how to resume non-emergency services safely, in light of the coronavirus risk.
It said a preliminary assessment suggests capacity will be affected by over 50% under the new system.
The HSE said it is likely to lead to longer waiting times for new patients, who are placed on the waiting list for urgent, routine and planned procedures in endoscopy units.
The executive said that during the pandemic, time critical emergency endoscopy procedures continued.
Additional reporting Fergal Bowers