The Department of Health described cancer waiting times in Northern Ireland as "unacceptable" after 200 people were not seen by a breast cancer specialist within its own 14-day target.
Officials said the latest figures showed a need to consolidate services into fewer locations - which is currently the subject of a public consultation.
During March this year, 1,387 patients were seen by a breast cancer specialist for a first assessment following an urgent referral for suspected breast cancer. Of those 1,187 (85.6%) were seen within the 14 day target, compared with 98% in February.
In March last year, every one of those referred was seen in the target time.
The Department of Health blamed "staffing gaps" in cancer services saying it has begun a public consultation on reshaping breast assessment care.
Margaret Carr, Cancer Research UK’s public affairs manager for Northern Ireland, said patients are waiting too long to get a diagnosis and start treatment.
“It’s clear that there is not enough staff to meet rising demand for tests," she said.
"The figures are unlikely to improve without significant investment in highly trained staff and transformation of diagnostic services.
“Improvements in these services, which include scans, colonoscopy or endoscopy, or blood or tissue tests, are vital as they help clinicians make a diagnosis of cancer.
“A focus on diagnostic services should be a major part of the work to develop a new cancer strategy in Northern Ireland and Cancer Research UK looks forward to working with the Department on Health on this.”
A spokeswoman for the department said the figures demonstrated the need for a concentration of services at fewer locations for "more reliable care, especially given the projected increase in breast cancer in the years ahead".
"A number of different trusts have fallen well short of the 14 day target in recent years, with staffing gaps the recurring cause," she said.
"That’s not the fault of the individual units or their hardworking staff. It’s because staffing numbers are currently spread too thinly across the province. Smaller units are particularly vulnerable to staffing crises, for instance when a colleague falls ill or takes a job elsewhere."
In March, 314 patients begun their first treatment for cancer following a urgent referal. Some 61.8% started treatment within 62 days, compared with 60.2% in February 2019. This compares with 75.4% in March 2018.
In March 2019, 2,052 new referrals for suspected breast cancer were received, this compares to 2,096 in February 2019 and 1,977 in March 2018.
Of those new referrals in March 2019, 1,424 (69.4%) were classified as urgent.